Yes, it's justified, with the usual caveats that translators never have access to words with the identical semantic domain in the target language as in the source.
hesed
The semantic domain of hesed can be understood as loyalty, kindness, goodness, etc. It has a broad range of meanings, but is most often translated as "loyalty" due to it's reciprocal nature The idea of reciprocity is core to the meaning of hesed.
Here is the Theological Dictionary of the Old Testament (which I can't recommend enough for anyone interested in word studies. It's absolutely invaluable)
d. On the basis of the passages already cited, we can make another
important observation. It is often stated expressly that the one who
receives an act of ḥeseḏ responds with a similar act of ḥeseḏ, or at
least that the one who demonstrates ḥeseḏ is justified in expecting an
equivalent act in return. Abimelech, for example, having taken in
Abraham as a guest, asks him to show the same ḥeseḏ to his host and
the land where he has sojourned (Gen. 21:23); Abraham so swears, and
this agreement is called a covenant in v. 27. The harlot Rahab
likewise asks the Israelite spies to show the same ḥeseḏ to the house
of her father as they received from her; they, too, swear to do so
(Josh. 2:12, 14). Just as David asks Jonathan for an act of ḥeseḏ, so
Jonathan also asks ḥeseḏ of David (1 S. 20:8, 14f.); once again this
is viewed against the background of the Yahweh berîṯ between the two.
And after the death of Jonathan, David fulfils the promise of ḥeseḏ in
the person of Jonathan’s son Meribbaal (2 S. 9). The mutuality of
ḥeseḏ is also mentioned in 2 S. 2:5f., where David, among other
things, promises to do good to the men of Jabesh-gilead because they
had shown ḥeseḏ to Saul and buried him. Similarly, David’s declaration
of sympathy upon the death of Nahash, which is called ḥeseḏ, is
occasioned by the fact that Nahash had previously shown David ḥeseḏ (2
S. 10:2, kaʾašer, par. 1 Ch. 19:2, kî). And David’s request to Solomon
to show ḥeseḏ to the sons of Barzillai and make them his table
companions is based (kî) on the fact that they had previously so
treated David and taken him in when he was a fugitive (1 K. 2:7).
There are a few other passages that, although they do not explicitly
mention this mutual demonstration of ḥeseḏ, nevertheless imply it in
their context. In Gen. 19, we must remember that Lot took in the
strangers and fed them. In Gen. 40, Joseph’s interpretation of his
fellow prisoners’ dreams justifies his request. The fact that the man
from Bethel showed the spies the way into the city earned his claim to
ḥeseḏ (Jgs. 1:24). And because the Kenites had previously shown ḥeseḏ
to Israel, Saul’s treatment of them is correspondingly friendly (1 S.
15:6). That it is among the ethical norms of human intercourse to
return ḥeseḏ that has been received is clear from the emphatic
statement that the Israelites did not show ḥeseḏ to the house of
Jerubbaal-Gideon, even though he had done much good to Israel (Jgs.
8:35), and that Joab did not remember the ḥeseḏ that Jehoiada had
shown him (2 Ch. 24:22). In view of the impressive evidence for the
mutuality of ḥeseḏ, we may venture the conjecture that even in cases
where the context does not suggest such mutuality it is nevertheless
implicit, because we are dealing with the closest of human bonds. In
the case of Abraham and Sarah (Gen. 20:13) as well as Orpah/Ruth and
Mahlon/Chilion (Ruth 1:8), it is the relationship between husband and
wife; in the case of Israel and Joseph (Gen. 47:29), it is father and
son; in the case of Laban/Bethuel and Isaac (Gen. 24:49), it is next
of kin; and in 2 S. 16:17, it should be noted that Hushai’s
relationship to David is called that of a “friend” (rēaʿ).
Zobel, H.-J. (1986). חֶסֶד. G. J. Botterweck & H. Ringgren (Eds.), D. E. Green (Trans.), Theological Dictionary of the Old Testament (Revised Edition, Vol. 5, pp. 47–48). Grand Rapids, MI; Cambridge, U.K.: William B. Eerdmans Publishing Company.
What complicates the translation of Proverbs 3.3 is that we have a hendiadys or an expression built from two words. Back to TDOT:
When we finally turn to phrases using ḥeseḏ, we find ḥeseḏ weʾemeṯ 4
times, all in the book of Proverbs (Prov. 3:3; 14:22; 16:6; 20:28; cf.
also 20:6, with ʾemûnîm). That we are in fact dealing with an
hendiadys can be seen from 16:6; only the first noun is preceded by
the prep. be, which thus applies to the phrase as a whole, a single
concept meaning “lasting, constant ḥeseḏ.” Such ḥeseḏ atones for
iniquity (16:6) and protects the king (20:28). Those who steadfastly
practice ḥeseḏ find favor with God and mankind (3:3f.). Whoever does
good will find lasting ḥeseḏ (14:22).
Zobel, H.-J. (1986). חֶסֶד. G. J. Botterweck & H. Ringgren (Eds.), D. E. Green (Trans.), Theological Dictionary of the Old Testament (Revised Edition, Vol. 5, p. 50). Grand Rapids, MI; Cambridge, U.K.: William B. Eerdmans Publishing Company.
In this case, the two words tend to have a single meaning and should be translated as such rather than each word individually.
ESV: "steadfast love and faithfulness"
Pr 3.3: Let not steadfast love and faithfulness forsake you; bind them around your neck; write them on the tablet of your heart.
Pr 14.22: Do they not go astray who devise evil? Those who devise good meet steadfast love and faithfulness.
Pr 16.6: By steadfast love and faithfulness iniquity is atoned for, and by the fear of the LORD one turns away from evil.
Pr 20.28: Steadfast love and faithfulness preserve the king, and by steadfast love his throne is upheld.
(Note hesed occurs by itself again in 20.28b)
The LEB has "loyalty and faithfulness". Given that this idiom refers to a "lasting, constant hesed", a translation as "loyalty and faithfulness" makes sense, although I would prefer "faithful loyalty" or "steadfast loyalty", and think ESV has the nicest approach with "steadfast love and faithfulness"
A translation of "loyalty and faithfulness" makes sense in the broader context of Pr 3.3, as "bind them around your neck, write them upon your heart." is also a reference to faithfulness or constancy.
Looking at the 3 other occurrences of this hendiadys, in Pr 14.22:
Have they not erred, those who plan evil?
But loyalty and faithfulness belong to those who plan good.
Again, this parallelism constrasts those who err (e.g. go off course) with those who are steadfast and constant in their loyalty.
And in Pr 20.28:
Loyalty and faithfulness will preserve a king,
and he is upheld with the righteousness of his throne.
Obviously kings are preserved by the steadfast loyalty of their subjects, but the point here is that the steadfast loyalty of the king to YHWH is needed, but the notion of constant loyalty as of a subject to a king is appropriate here.
Then we come to Pr 16.6, which is the only place where this hendiadys might seem a bit unusual:
By loyalty and faithfulness, iniquity will be covered over,
and by fear of Yahweh one turns from evil.
My interpretation is that by the loyalty and faithfulness of the messiah, iniquity will be covered over (or atoned for). That is, it's not our own loyalty and faithfulness that atones for iniquity. But at this point we are veering off the path of translation and into exegesis. But as a translation, I think this is certainly justified, although we can argue if other translations might be better.
