Briding Amplifier

You can't bridge one channel. Bridging is combining the power of both channels into one.  If you are saying he is going to run 2 subs on one channel and one on the other, they will get roughly the same amount of power because the channel with the 2 subs with have half the resistance of the other ( if wired in parellel), causing it to get 2 times the power to be split between 2 subs.  However, they will have a higher THD.  However, this is not a very good way to run the subs. But it will work if the amp is stable to 2 ohms on each channel.

If I were your friend, I would save up for a mono sub amp that can run safely to one ohm, wire the 3 subs in parellell to get a 1.33 ohm load (if they are SVC) and run them like that.  He will be way more satisfied with the results.