ME1 & 2 are on Steam. ME3 is only on Origin, unfortunately.
How are they RPGs? Two ways, really--
1) As previously mentioned, you have a party of characters. In RPG tradition, when you go on missions and quests, you select who comes with you. Different characters fit into archetypal RPG classes with their powers. In the ME universe, the equivalent of magic is Biotics and Tech; Biotics is a bit like the Force, where you can manipulate space and warp time and things like that. Tech is hacking technology and things like that; that all plays into combat. There's combinations you can do by ordering your allies to act in a certain way; Hey, Biotic, lift that guy off the ground, and then I'll use my Biotics to throw him backwards, things like that.
Within all these powers you level up, and there's a basic tech tree that branches, so you can get different variations on powers depending on how you level them; do you want them more or less offensive, etcetera. You create your own character and you choose their class; do you want to be a soldier, who just uses guns (basic third person shooter gameplay) or do you want other powers?
2) Story stuff. I think in terms of playing a role, this is the bit that really draws people to ME. The choices you make are big, have an effect, and carry from game to game. I was seeing consequences to choices I made in 2006 or 2007 in 2012 in the first game when I played Mass Effect 3, which was pretty amazing. The choices you make really attach you to the characters and your personal story, which will be different to everybody else's, which I think is great.
The core gameplay controls like a third person shooter, but there's a lot of RPG stuff under the hood. I've seen people do playthroughs of ME1 where they don't use guns at all and ONLY use the powers, so there's a lot of scope there.
Look for a copy of ME1, but it may be the case now that the cheapest way to buy it is to buy the trilogy box set.