The three pin plugs are called XLR sockets they are for microphones.

How have you been plugging in your mics and what sort of mic are you using ?

This is another potential problem if you are plugging your mics into a guitar socket which may be why they sound nasty and quiet however the guitar should sound OK. Have you tried plugging in an electric or have you just been running acoustics through it and if so, how have you been hooking up the acoustics ?