The i3, i5, and i7 chips support 3 displays. I don't think USB anything changes that.
[ addition Oh, yes an external USB 3 display adapter ... but you have to be concerned with total bandwidth consumption as a high res USB display adapter can be a hog. you really need 10gb thunderbolt for this vs 5gb USB]
The i3 graphics is the 4200 graphics with 20 execution units @ 200 - 850 mhz
The i5 is 4400 with 20 EU @ 200 - 1100 MHz
and i7 is 5000 with 40 EU @ 200 - 1100 MHz
Here's an Intel hard to read doc with details. I skimmed it quickly and didn't see what resolution limits were in play.
Forgot to mention I'm using a Targus USB 3 docking station which I chose because it lays flat vs the standup Pluggable units.