Multiple Amp Install

You'll probably start by changing the power and ground wire sizes to accomodate 3 amps now instead of 1.  Add up the total watts and upgrade to the proper size wiring.  Upgrade the power wire fuse as well.  There are a lot of distibution blocks to pick from that will accept the power wire and split wires out to the amplifiers.  Get one that is fused, and place fuses in it that are appropriate for the size wires that are feeding the amps.

Often, mostly for a clean install, a grounding block (unfused) can be used too.  The ground wires for each amp would match the power wires that feed them, and the main ground to chassis would match the main power wire gauge.

A lot of people have no problem splitting a remote to 3 amps.  Mathematically, the sum of the draw should be well less than the amount that would harm the deck.  But I've often heard that for more than two amps you should use a relay to split the turn on.  If you want to use the relay, use a common Bosch and connect as shown here.  Turn on wires can always be connected to the car's 12 volt switched supply as well as the head unit.  The difference would be that the amp would turn on when the car is turned on.  Connecting one amp that way, with two split to the head unit, would get you around that "3 amps need a relay" rule without having to use a relay.

Look to the left column here for more info on power/ground wire sizes appropriate for the current they have to handle.