Whether it will run three monitors is a question better-suited for the people from whom you are purchasing the computer (or the folks who make the GPU).
The GPU matters very little for core After Effects. The out-dated and out-moded ray-traced renderer is the only built-in thing that really used the GPU at all. See here for more info.
For a useful talk about how After Effects and Premiere use system hardware, watch this event.
For information about components to purchase for After Effects and Premiere, see here.