Label the amps A, B, and C. Lows, mids, and hi's. Connect A left to the bass cab on the left of the stage. Connect A right to the right side of the stage bass cab. Do the same with the other two amps. 6 amp outputs to 6 speakers. All the left outputs to all the left speakers, all the right outputs to the right speakers. If the amps are true stereo, center them. If not, set the gains the same.