This, I also strongly suggest you get the first two, or at least the second one if you have a PS3. It's an experience you can't miss out on if you are into sci-fi.Definitely play the the first 2 games before playing Mass Effect 3, if possible. It's not just about knowing the story, it's about all the choices you have made during the first 2 games all carry over to the third one and have drastic effects on how things play out. Each game's combat system is different too, and they're all great in their own way. Definitely one of the best gaming experiences I've had, all three titles can easily go on my top 20 games of all-time.
Note that ME1 is not available on PS3, but I think ME2 comes with an interactive video that gives you a nice recap of the first game, and allows you to make the major choices there.