It sounds like you have a good set of amps but what will you be using them on? What kind of subs will you be running and what impediance will the amp be loaded to?
If you are running the 3000 at 1ohm it could be possible that you will need a biger alternator but if you are running it at 4ohm or even 2ohm you might only need a new battery and at least a 3 farad cap but bigger is always better so if you wanted to up grade your alternator.....that would be the great.
Another thing that most people forget about is the factory wiring in the vehicle. If you look at the factory ground and power wires coming out of the battery posts, they are most likely 8 gauge crap wire. If you want to make sure you get the power to your amps, change all your factory wire ie: alternator to battery, ground from battery to chassie and engine, etc. What this means is, if your using 4 gauge wire to your amp from the battery, use the same wire and upgrade any wire you see coming out of your battery posts.
Have fun building the system.