the reason the 2 15s sounded better than the 3 is because you have 3 big subwoofers that want to move a lot of air, the 3 15s in the trunk are cramped and have no air to move, plus the 2 in one box and 1 in a single, the 3 subs probably don't all have the same air space per sub so they are not firing simultaneously causing air turbulence and the subs are canceling, you would probably sound better if you made a good ported box and ran 2 of them underpowered at 2ohms with all gains and settings set correctly