Short Answer: "Generally it is the only translation" (but it is complicated)
First, there are two (three?) different words in the references you give. The Nephilim (נְפִילִים; a word only ever found in plural form in OT) only appears three times in Gen 6:4 and Num 13:33 (twice). The word in 1 Ch 20:8 (also 1 Ch 20:6 and 1 Ch 8:2; cf. also 1 Ch 4:12) is Rapha (רָפָא; singular), also only found these three times (though that depends on interpretation of homonyms, see below). In Josh 15:8 is the word Rephaim (רְפָאִים; plural), which is always found its eight times (also Josh 18:16; 2 Sam 5:18, 22; 23:13; Isa 17:5; 1 Ch 11:15; 14:9) in connection with the word for valley, Emeq (עֵמֶק), hence in the translations either translated "valley of giants" or transliterated "valley of Rephaim."
Second, many major lexicons essentially uphold the word "giant(s)" as the translation in these spots, though you will find some differences as to what "giant" even should "mean." Many choose not to translate the words because of issues in knowing what translation is best, and so they transliterate the Hebrew sounds into English (hence, Nephilim and Rephaim).
Of the Nephilim (נְפִילִים):
(1) The Hebrew and Aramaic Lexicon of the Old Testament (Ludwig Koehler, Walter Baumgartner, M. E. J. Richardson, and Johann Jakob Stamm) gives some varying thoughts on the word:
giants, arising from miscarriages or hurled down from heaven (Koehler
Mensch 38) :: H. Gese Vom Sinai zum Zion (1974):11047: those who have
fallen heroically in a battle, the giant-like early inhabitants of
Palestine Nu 1333aβ (gloss, which links them with the עֲנָקִים, GnAp
2:1; Fitzmyer GenAp2 81), with a mythical origin Gn 64, Sept. γίγαντες
(A. Schmitt ZAW 86 (1974):152f); Morgenstern HUCA 14:76ff; Albright
Steinzeit 295; Humbert Fschr. W. Vischer 70ff, 76; Reicke-R. Hw. 1601;
Stolz BZAW 118 (1970):97; Westermann BK 1:510f.
(2) Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (Francis Brown, Samuel Rolles Driver, and Charles Augustus Briggs) essentially just gives "giants" with no real expanded commentary.
(3) Gesenius’ Hebrew and Chaldee Lexicon to the Old Testament Scriptures. (Wilhelm Gesenius and Samuel Prideaux Tregelles) gives more discussion, but also a key point about etymology (emphasis added), which explains some variation of opinion on what "giant" refers to:
giants, Gen. 6:4; Nu. 13:33. So all the ancient versions (Chald.
נִפְלָא the giant in the sky, i.e. the constellation Orion, plur. the
greater constellations). The etymology of this word is uncertain. Some
have compared نَبِيلُ, نَبِيلَةُ, which Gigg. and Cast. render, great,
large in body; but this is incorrect; for it means, excellent, noble,
skilful. I prefer with the Hebrew interpreters and Aqu. (ἐπιπίπτοντες)
falling on, attacking, so that נָפִיל is of intransitive
signification. Those who used to interpret the passage in Genesis of
the fall of the angels, were accustomed to render נפילים fallers,
rebels, apostates.
(4) Dictionary of Biblical Languages with Semantic Domains: Hebrew (Old Testament) (James Swanson) notes the theological debate specifically (of which there are other variations than the two he notes):
Nephilim: a renown race of giants (Ge 6:4; Nu 13:33+), note: whether
this race was a spirit-human race, or godly-ungodly race is debated
among theologians
**(5) Article “1393 נפל.” (Milton C. Fisher) in Theological Wordbook of the Old Testament (Edited by R. Laird Harris, Gleason L. Archer Jr., and Bruce K. Waltke) gives more discussion about that uncertain etymology:
giants, the Nephilim (Gen 6:4; Num 13:33, only). While some scholars
attempt to relate this term etymologically to nāpal I via the noun
nēpel “untimely birth” or “miscarriage” (as productive of superhuman
monstrosities), a more likely reconstruction is the proposal of a root
nāpal II, akin to other weak verbs, pûl II “be wonderful, strong,
mighty,” pālāʾ “be wonderful,” and even pālâ “separate, distinguish,”
pālal “discriminate.” This pattern of semantically related groups of
weak verbs with two strong consonants in common is a notably recurrent
phenomenon in Hebrew lexicography. Actually, the translation “giants”
is supported mainly by the LXX and may be quite misleading. The word
may be of unknown origin and mean “heroes” or “fierce warriors” etc.
The RSV and NIV transliteration “Nephilim” is safer and may be correct
in referring the noun to a race or nation.
Of Rapha (רָפָא) and Rephaim (רְפָאִים):
These words seem clearly related as far as the Hebrew root goes (רפא). However, not all the lexicons necessarily seem to link what would seem to be the singular and the plural forms.
(1) The Hebrew and Aramaic Lexicon of the Old Testament (Ludwig Koehler, Walter Baumgartner, M. E. J. Richardson, and Johann Jakob Stamm).
For Rapha gives the following, and sees three different referents:
רפא short form “he (יהוה/אֵל) has healed” ...
1. ... Sept. (acc.) Βαθρεφαν, the designation
of a place or a clan in Judah, constructed according to the name of an
ancestor ...
2. a son of Benjamin, a Benjaminite clan named after an ancestor ...
3. [notes only Hebrew construction and reference in 1 Ch 20:6, 8]
For Raphaim, they discuss it under the entry for Emeq (עֵמֶק; valley), without any elaboration.
(2) Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (Francis Brown, Samuel Rolles Driver, and Charles Augustus Briggs)
For Rapha they simply list references, no elaboration.
For Rephaim, they give two entries (as there is apparently a homonym meaning "shades, ghosts"). The second relates to your question:
old race of giants (perhaps = I. ר׳, as extinct and powerless; v.
especially WRS in Dr Dt 2:11; or as shadowy, vaguely known,
SchwallyZAW xviii (1896), 127 ff. v. also Stal.c.);—ancient inhab. of
Canaan ...
Even their second definition notes the possibility of "shadowy, vaguely known," which would seem to indicate possible closer connection to the homonym.
(3) Gesenius’ Hebrew and Chaldee Lexicon to the Old Testament Scriptures. (Wilhelm Gesenius and Samuel Prideaux Tregelles)
Of Rapha (second idea, as it notes homonym also) he mainly simply gives the references, but notes of 1 Chr 20:4 "the ancestor of the Canaanitish nation of the Rephaim."
Of Rephaim, he treats that both under the entry for Rapha and that of Emeq (עֵמֶק; valley). Under the first, he considers it related to the homonym idea and htus meanign "shades living in Hades," under the valley entry stating "('the valley of Rephaim'), southwest of Jerusalem, towards the land of the Philistines."
(4) Dictionary of Biblical Languages with Semantic Domains: Hebrew (Old Testament) (James Swanson)
For Rapha:
- son of Benjamin (1Ch 8:2+) 2. one from Gath (1Ch 20:6, 8+), see also 8335; note: some sources translate as a common noun “giant;”
also, a part of a compound name, Beth Rapha
For Rephaim he relates it to the homonym idea:
the dead, i.e., a class of beings that are the spirits of the
departed, with a focus on the beings as ghost-like, not having
material substance
He also notes the valley so named: "broad relatively flat valley about 2 mi. WSW of Jerusalem temple mount"
(5) Article “2198 רָפָה.” (William White) in Theological Wordbook of the Old Testament (Edited by R. Laird Harris, Gleason L. Archer Jr., and Bruce K. Waltke).
There is much discussed, both on the side of each word relating to "Ghosts of the dead, shades" and "Giants, Rephaim," with the latter considered for the references you gave. Of the giants, he states in part (note my last emphasized phrase):
A term of Ugaritic origin, the designation for one of the most
primitive pre-Semitic peoples of Palestine. According to the Ugaritic
myths and legends the hero Danel (no relationship to the biblical
Daniel), and the champion of Baal are both called “Rephaites.” Whether
this is a folk etymology or a term based upon some ancient historical
term is unknown. The OT historians used it to describe the
pre-Israelite inhabitants of Palestine. Deuteronomy 2:20 mentions that
the Ammonites called them the “Zamzummim” but no data is extant on
this name. The versions are confused in translating the term.
