i know that the exhust system in honda include 3 cans under the car. my question is if i remove the first (under passenger seat) and the middel and let the last one( on the back ) is this affect at the power ?

the one closest to your motor is the cat, the next one is the resonator, and then the muffler. i removed my resonator and i have a 2001 eclipse muffler and it now sounds like a bumblebee. i dont like it, so im going to cut my cat off, and put a glasspack (cherry bomb) in its place to keep the flow through and a deep sound. but to answer your question, yes it will make it more flow through.