That being said, here's what I think:
The three mikes got shorted out. Could have been a freak power surge, or something weird about those three channels that took care of itself on your mikes. If they don't work with any board now, they're dead and need to be fixed or replaced.
You should be able to clean contacts if you feel comfortable taking your amp apart to do it. Just taking the contacts apart and putting them back together again might be enough to get enough gunk off them to get it working again. You could also try a q-tip with rubbing alcohol on it to carefully clean the parts. If it still shorts out after that, there's something broken somewhere and you'll need to take it in.
-Psiclops