The suit he uses in Avengers is a new one.
So in Iron Man 3 the suits would presumably be:
Mk. VII (First appeared in Avengers)
Iron Patriot (Supposedly first appeared as MK II in Iron Man)
& Mk VIII (Gold plated/Extremis[first seen in Iron Man 3])
You're right, Mk.7's the one we see at the end of Avengers, I suppose it was made to endure more firepower while featuring that mid-air suit-up mode that I think wasn't in the previous ones? Design wise there's little difference tho. And I think Mk.7's no longer functioning after the Chitauri attack.
Evolution of the Iron Man armor
Mark I is easily distinguishable, nothing more to add to that (except maybe an extensive comparison with Iron-Monger)
The
Mark 2 suit is the silver, and I'm guessing has the same features as mark 3 except for solving that in-story "icing-problem," making it an ideal ground-suit. But I think, as with most of the updates from here-on out, the feature for this was that it was streamlined and was capable of flight as well, though probably lacking all the greater fire-power that the next suits have at this point.
Mark 3 is essentially the last one with a different alloy to withstand better flight. It's got more fire-power, is connected with the "Seraphim satellite" (???) and arguably more durability. But design wise, it retains the basic designs save for the hot-rod red and gold colour-swap with Mark 2:
(I'm looking at the pattern on the arms, the chest, and the mid-section)
In addition, of course, Mark 3 masters the flight and has those brilliant flaps to keep this grounded in reality:
(also, yes I wanted to post more pictures

)