3 passive sub at 8 ohm imp in parralel will be 2.6+ impedence.Most of the professional sum amp can take up to 2 ohms load per channel(check the spec of the Amp)eg: you should be able to connect max 4 subs in each channel(match the watts pwr to amp)
Your amp must be bridgeable first of all and it should show you on the amp, most do but you put the + on ch. 1 and the - on ch. 2 sometimes the othrr way around.
If you amp is bridgeable you run + sub wire to ch 1 + then the -sub wire to the - of ch 2 of the amp. The same with the other sub + ch3 - ch4 that's how to bridge your amp.
First, the 2 ch amp has to be bridgeable. In the "Pro Audio" world, PA, Sound System etc... some amps are easier to bridge then others. The easy amps are the one that have "Binding Post" for speaker...