Deutéronome 17:6

Upon the word of two witnesses or of three he shall die who is guilty of death; they shall not be put to death on the word of one witness.

or three witnesses Sa‘adya Gaon interprets the passage to read “three (people who accept the testimony of the) witnesses”. Some say that if two witnesses contradict two other witnesses, then a third party can reconcile the conflicting testimony, whether it lead to a conviction or to an acquittal. Others say “two witnesses” if they be scholars, but “three witnesses” otherwise. Thus you see that we require, perforce, the words of the Tradition [Makkot 5b].

AT THE MOUTH OF TWO WITNESSES, OR THREE WITNESSES, SHALL HE THAT IS TO DIE BE PUT TO DEATH. “If the evidence can be sustained by two witnesses, why did Scripture specify ‘three?’ It is to liken three to two: just as the evidence of two witnesses forms a single unit of testimony, so does the evidence of three, constitute a single unit of testimony and they are not subject to the law of plotting witnesses [which declares that whatever they plotted to do to their brother should be done to them] unless all three are proven to be plotters.” This is Rashi’s language. So also the principle [of a single unit of testimony] applies if one of the witnesses is found to be a kinsman [and hence ineligible to testify] or to have some other disqualification — the testimony of all is void. If they are proven to have been plotting witnesses, [in which case] all of them are subject to the death-penalty or payment of money [as the case may be, and the third witness cannot claim immunity because of his argument that even had he not testified the evidence of the first two witnesses was sufficient to sustain the charge], since the main intent of Scripture was to liken the evidence of three witnesses to that of two in every respect, according to the opinion of our Rabbis.
And the Gaon Rav Saadia, p. 521, Note 74. Rav Saadia’s comment is quoted here by Ibn Ezra. said, by way of the plain meaning of Scripture, that the interpretation of the verse is as follows: “two witnesses, or three judges who [officially] accept the testimony of the two witnesses.” thus means: “upon the testimony of two witnesses who appear before the entire court of twenty-three judges, or by the word of the three judges who officially accepted the testimony of the witnesses and later relayed it to the entire court — shall the one that is to die be put to death. Ramban will refute this interpretation of the Gaon, for there is in the verse etc. But there is in the verse no mention of acceptance of the testimony; instead, it refers explicitly to witnesses. But it seems to me that the Gaon erred in his [statement of this] law, for testimony in capital cases cannot be accepted except in the presence of a Sanhedrin of twenty-three judges [and not, as Rav Saadia holds, before only three judges]! Rather, the plain meaning of the verse is that the condemned man shall be put to death at the mouth of two witnesses if no more are there, or at the mouth of three witnesses if three witnesses are to be found there. Scripture thus states: when it will be told thee, and thou hear it you are to inquire diligently into the matter by means of the testimony of all the witnesses found there. Thus if we heard that he transgressed in the presence of three people, we should send for them and have them all come to court and testify. The same law applies to a hundred, for by hearing the words of them all, the truth will become clear. But if not more [than two] were present, or they [the others] left and are not to be found, then two are sufficient.

על פי שנים עדים או שלושה, even if the charge is idolatry, the entire testimony is disqualified if one of the witnesses is found to be inadmissible. We do not rule that seeing there are a sufficient number of reliable and legally admissible witnesses left, we proceed with the trial on that basis.

או שלשה עדים, “or three witnesses.” Seeing that in Exodus 23,2, the Torah had decreed that a majority opinion in disputes is to be accepted, it was necessary to tell us that when it comes to testimony, numbers do not count. In practice this means that if one of the three witnesses in litigation has been found as not trustworthy according to Jewish law, we do not use the fact that the remaining two witnesses are trustworthy as believable, seeing that they form a majority. The entire testimony is declared as null and void because one of the witnesses was disqualified. The same applies if there were 100 witnesses and only one of them was disqualified. If however, two witnesses had come forward concerning a certain court-case, and some relatives of the accused wanted to join these two witnesses,-presumably in order to make the two witnesses useless,- this is not allowed as the relatives had never meant to accuse their kin in the first place of the sin he is accused of. Unless we applied this rule, we would never find two qualified witnesses that could testify to the validity of a marriage ceremony. There are always numerous relatives of the couple present, and anyone could succeed in invalidating that marriage if he so chose.