First and foremost...
Gain: It is not a volume knob. Gain "matches" the input of the amp to the output of the head unit/stereo.
Alright, now the issue. Most likely your amp is going into protect because it's clipping. A clipped signal is basically a sine wave. Not only that, but a speaker "load" actually changes based on temperature, frequency it's playing, etc. So your amp isn't "seeing" a 1ohm load. It's all over the place...
In addition, you need to verify that all 3 subs have 4ohm coils. After you verify that your speakers are 4ohms a piece fix your gain. Clipped signals destroy pretty much everything in car audio. If you ever had a friend and their equipment failed...most likely it was from a "clipp'ed" signal. 90% of car audio failures are from clipping. Owners are just ignorant as to what happened and it's always "over powered" this or "under powered" that...Luckily you're a smart dude!! You have a an amp w/ protection mode
Sorry if I came off as brash...I just like to be direct
