3 amp question

You can do that through a distribution block, same as you would use a distro for the power wire inputs.  The ground wiring must be as large as or larger than the power wire for each amp.  Don't run the ground from one amp to the ground terminal of a second amp.  I prefer to run separate grounds from each amp, and connect at the same chassis grounding point, or at least within an inch or two from each other.  But if you use a distro for ground, you could use a simple unfused block like this one for the grounds.

For example, if you have a main power wire from the battery at 4 gauge, then split out with 8 ga. to each amp through a distro, then do the same in reverse for grounding, ending up with a 4 ga. wire to the car chassis metal.