D'varim 17:6 tells us that "at the mouth of two witnesses, or three witnesses" a transgressor can be put to death. The mishna in Makkot 1:7 (and following) asks why it says "or three" if two is enough (I've long wondered that too), and derives some conclusions about groups of witnesses from this passage. However, the mishna and g'mara also says the following things:
- The testimony of the third is superfluous. It can't help.
- It's not insurance, because if any member of a group of witnesses is disqualified, the entire group is. (For example, one of the three is a relative; even though the other two are not and two witnesses can convict, their "association" with the third voids their testimony.)
- All witnesses in a group are punished if they are found to be conspiring witnesses. Even when one doesn't intend conspiracy, there's always room for differences in testimony to result in such a charge. Being a witness is a little risky.
Given all that, why would more than two ever testify together? Is there a requirement that everybody who was there together testify even if they don't want to, and that's how you can get more than two witnesses? If not, why would a third (or fourth or hundredth) person choose to join a group of witnesses?