Explanation #1: According to Rashi, the Gemara understands that there are nine. How is that? One of the litigants picks one Beis-Din - that makes three; the other one picks a Beis-Din - that makes six, and they both pick a third Beis-Din - that makes nine. Hence the Gemara asks that three will suffice?

Refutation: But that is not correct - since that is not at all the implication. If it was, then the Gemara ought to have asked explicitly that there is a contradiction here between the Reisha, 'Dinei Mamonos bi'Sheloshah, implying three, and the Seifa, which requires nine.

Explanation #2 (part 1): It therefore seems that the Gemara new at the outset what it ultimately concludes - that each litigant is permitted to disqualify his opponent's Beis-Din, and that in the end, it is the third Beis-Din that actually judges (like the Metaretz answers); And the Kashya is on the Lashon of the Mishnah, which states 'This one picks ... and that one picks, as if it was obligatory for each one to do so. That is why the Gemara asks 'What does it mean "Zeh Borer Lo Echad ... "? Do they really have to do that?

Explanation #2 (part 2): And the Gemara answers that the Mishnah speaks when that is what they did, but does not mean that it is obligatory to do so.

Explanation #1: Rashi explains that they judged in the same town.

Refutation: But that is not correct, seeing as we have learned in Eruvin that Rav Chisda declined to issue rulings in front of Rav Huna (i.e. in the same town) even something as simple as 'cooking an egg together with a milk-stew', and he would certainly not rule in money-matters in front of him?

Explanation #2: It therefore seems that they lived more than three Parsa'os of each other (and it would not be a big Tircha for the creditor to go from one to the other).

Explanation #1: The Rivan explains that the litigant who is found guilty will pay willingly, and he will declare that the Dayanim certainly judged him fairly, seeing as he himself picked one of them, and had been able to judge in his favor, he would have done so.

Explanation #2: The correct explanation, however, seems to be that since each litigant picked his own Dayan and the two of them then picked the third, the final ruling is bound to be absolutely correct, without any injustice. This is because the judgement will be balanced, since those who are searching to declare either party guilty do not exceed those who are searching to declare him innocent.

Question: Why does the Gemara mention Rav Dimi?

Answer: If not for Rav Dimi, we would not have been able to ask from the Mishnah. Why is that? Because we would have thought that the Mishnah is speaking where he accepted his father as two Dayanim, which is a double fault, and that is why R. Meir holds that he can retract, but if it was a single fault (such as in the case of Rav Dimi), R. Meir would not permit him to retract. So the Gemara cites Rav Dimi, to teach us that R. Meir permits him to retract even when there is only one fault, thereby justifying the Gemara's question 'Tanina!'

Explanation #1: This is Rashi's version of the text, according to his second Lashon, that 'the Rabanan hold 'Tzarich Levarer'. Should a litigant claim in Beis-Din that he has two pairs of Kasher witnesses, then he is obligated to support his claim and to bring two pairs of Kasher witnesses. Failing which, he loses his case. That being the case, he cannot disqualify the first pair, because once they are disqualified, he cannot claim his debt with the second pair only, seeing as he was unable to support his claim and bring two pairs of Kasher witnesses.

Refutation: Rabeinu Tam queries this however, based on the Gemara in Perek Get Pashut, where it is implied that even those who hold that he needs to support his claim, that is only insofar as the Beis-Din will order him Lechatchilah, to do so. In the event that he is unable to however, he will not lose his claim?

Explanation #2: Rabeinu Tam therefore accepts Rashi's first Lashon, according to which R. Meir holds 'Tzarich Levarer'. Consequently, he is able to disqualify the first witnesses, seeing as he does stand to lose anything if they are disqualified. This is because when Beis-Din instruct him to clarify his words, and he complies with their instructions; he will not lose anything should the first set of witnesses become disqualified, seeing as his testimony stands with the second set.