The scribes at Qumran had there own scribal practices which were distinct from those practiced during the second Temple period, and in the subsequent period which saw the copying of the Temple scroll in Tiberius (1 of 3, the only to be returned by Rome to the Jews of this city in the Galilee), and eventually transmitted to us by Moses Ben Asher in the Masora. For now lets deal with the differences between Masoretic/Tiberian scribal practices and Qumran scribal practices. Some texts such as the Great Isaiah scroll resemble the Masora more than others found at Qumran: "The version of the text is generally in agreement with the Masoretic or traditional version codified in medieval codices, such as the Aleppo Codex, but it contains many variant readings, alternative spellings, scribal errors, and corrections...Strictly speaking, the number of textual variants is well over 2,600, ranging from a single letter, sometimes one or more words, to complete variant verse or verses"(source: http://dss.collections.imj.org.il/isaiah)
As for the LXX, "Unique agreements between the LXX and the
Qumran scrolls...abound in all books of the Bible"
Source: Tov, Emmanuel. Septuagint, Scrolls and Cognate Writings: Papers Presented to the International Symposium on the Septuagint and Its Relations to the Dead Sea Scrolls and Other Writings (Manchester, 1990. p. 18. Accessed at http://khazarzar.skeptik.net/books/brooke01.pdf
Generally, Tov argues that the Dead Seal Scrolls reflect original Hebrew variants/manifestations/proto-forms (vorlage), rather than LXX mistranslations. Additionally, he states that "there are hundreds of examples which enhance the credibility of the LXX as a text critical tool in biblical studies" He goes on at some length, which is worth quoting: "The LXX should indeed be taken seriously as a tool for the textual criticism of the Hebrew Bible. In spite of known
trends of exegesis in the translation, of inner-translational corruptions and of our own ability to get back to the Hebrew text underlying the
translation, much of what has been done so far in the area of retroverting the Vorlage of the LXX is now supported by the Qumran finds" (ibid p. 21). Here, retroverting refers to reconstructing a variant Hebrew reading, rather than reconstructing the possible translation error.
However, "It is often difficult to know whether a reading of the LXX which differs from the MT should be reconstructed as a deviating Hebrew
reading or should be regarded as the translator's exegesis...If there is
little textual variation in a given unit, as in the case of the LXX and MT of Isaiah, the relation between these two sources on the one hand and a
Qumran scroll on the other is bound to be very similar. Thus all the Isaiah scrolls from cave 4, to be published by Prof. Ulrich, agree with the MT
and LXX almost equally. It is therefore often irrelevant to assess their closeness to either the MT or LXX" (ibid. 22-24).
I highly recommend reading Tov's paper found in the above source for a more in depth study on the LXX's relationship to the Dead sea Scrolls and the Samaritan Torah.