1st: The cap was an unnecessary expense and your money would have been spent much more wisely (and less of it, too, considering the average retail pricepoint on 1 farad "stiffening" capacitors!) going through the process of upgrading your "Big 3" underhood wiring.
If this is something unfamiliar to you then please take a few moments to read through (and actually absorb) the information
found here.
2nd: For that much amplification I would suggest getting a distribution block that accepts a 1/0awg cable input and splits into 3 or more 4awg outputs. The 1/0awg would feed from your battery to the input side of the distro and then the outputs would feed your three amplifiers. So you'll need some 1/0awg cable for main power then some 4awg cable for the feeds to the amps.
Remember you're going to have power and ground connections so you'll need to double the length of cable (as well as a second distro) you would have bought for the above breakdown of necessities.
Additionally you'll want an inline fuseholder to fuse the main power cable.
Depending on the type of distribution block you buy you may or may not need additional fuseholders to fuse each individual amp.
You'll need RCA interconnects to bring signal from your head unit to the amps.
You'll need some small guage cable for remote activation leads. Since you're running three amps I would also suggest using a 30a SPST relay to activate your amps after being triggered by the headunit's remote accessory activation output. It's cheap insurance against potentially burning the remote activation circuit in your headunit asking it to turn on too many things.
3rd: The cap, if used, would be fed power and ground from the distribution blocks then the power to the amps would be sourced from the cap while they would be grounded to the distros as well.
4rth: How much alternator (speaking about amperage output) does your car have?
5th: Power HX2s.
Yum.
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