Stock Deck Running 3 12s?

[captainrob34]

I'm trying to hook up my 3 12" jl audio w3's to the stock deck on my 95 LS400. Ive got a 600w aftermarket amp, but was wondering how i could hook all this up to the stock deck and keep that clean look. Is the stock amp only for the stock 8" sub or does it power the spearkers too? My new amp is a mono sub amp, so if the stock amp powers the speakers, i'd really like to keep that for everything but the subs. Any help on how i can wire my mono amp up to the stock deck is greatly appreciated.