The combat is really the only facet that can be said to play anything like Gears of War, and even then that's if you choose to play as a weapon-heavy class and ignore biotics, tech, and the control wheels. The mechanics are going to be pretty similar to the second game overall, despite the improved AI and more complex enemy class system.
Overall, the combat is really only one part of the game. There are still RPG elements: branching upgrade trees, more complex weapon and armor modding, dialog trees that determine moral orientation, relationship building, and most notably a new system where you have to gather and manage lots of details for a coming galactic war. If you didn't like the RPG stuff in the second game, you probably wont like it now, especially since it's more involving in some ways compared to ME2.
And personally, I don't see much point in jumping into the game in the third installment. BioWare is all about accessibility now, so they've made it relatively easy to do so if you're new to the game, but the writing really caters to fans of the series or, at the very least, those who have kept up with the events of the series. Mass Effect's strongest suit is the story and characters, and if you're not attached to either, all that's left IMHO is a fairly standard 3rd person shooting game with a fair amount of RPG elements. If you're just looking for a 3rd person shooter without the RPG stuff, there are better alternatives (like Gears of War).