how to install 3 subs to 1 amplifier

Well, heres a diagram of how to wire up three 4 ohms subs if thats what you have, BUT, your amp will need to be 1 ohm stable so give us some more details on what kind of stuff you have to work with.

ok heres what i got for subs and amplifiers

Ive got 3x  10'' kenwood KFC-2503 subs, 4ohms imp. 500w peak power and there single voice coil. Ive also got a Kenwood KFC-W2585 amplifier 450watts and a kenwood KAC-748 power amp aswell but dunno which amp to use to power the 3 subs.

Ive tried to hook them up all in parallel but it seems i dont get enough boom!

hooking the subs up in parallel will definitely fry your amplifier...  the KAC-748 is a 4 channel amp that is stable only down to a 4-ohm mono load and 2 ohm stereo.  Kenwoods are not knowing for powering subs to the extreme but if you are looking for bass you might want to look into purchasing a mono block amp. 

This will cost you a little but it will definitely get you the boom that you want and save you some $$$ in the long run.  If you continue to run your kenwood below 2ohms the transistors within will began to break down..