You got 3 subs? That's gonna be a funny lookin box
If you get one of those all in one kits it should come with almost everything you need (I got the biggest fosgate kit I could find, it was only short 1 ring connector and 1 large fuse). As for wiring it up, look at the paper that came with the cap to see how it should be wired, it's a pretty bad drawing but it's not too hard to figure out.
Basically it goes positive on batter to cap, pos on cap to amp(s) (the big amp kit comes with 2 boxes with splitters so your power and ground can be all be split up from one set of sources) ground all amps and cap to something in your car (I used seat belt bolt) wire up amps, charge cap, good to go!