A: If you required unanimity, the decision of the three would be at least a majority vote even if all five had been present. This is appealing, but is neither necessary nor advised.

An important issue is often an issue of timeliness, too. If you require (through a bylaw or special rule of order) at least three affirmative votes, you may put yourselves in a position where one member can stop the progress on an issue the other two at the meeting (and possibly the two absent members, too) endorse. In other words, one member could overrule the will of the rest of the board.

Another consequence of requiring three votes is that it would not allow any member to truly abstain, since his abstention would be like a negative vote. Abstentions are important since they allow members who do not feel knowledgeable about an issue to not vote -- certainly a better choice than voting ignorantly.

If the issue isn't urgent, then do consider postponing the question. But don't let a less-than-full board stop you from acting in the interest of your school.