You must play all 3 games (in order). Some games you die at the end (IE: Fallout-3) which is no-big-deal because you're on your OWN. I played F3 for 9 months. Alternatively, this game ME3 is different in that you get to know and build fantastic friendships & relationships with TWENTY (20) different personalities (various humanoids) that you will know intimately -- who work with you and save your butt many times.
Even though you mentally agree this is simply a fictional game -- you will find that you've been emotionally addicted to the characters -- [very] unlike any of the some 200 games I've played. This has a way of getting into your soul whether you like it or not.
If all games (ME1, ME2, ME3) are played correctly, you can save all 20 of your team mates at the end. Play stupid, and most of them will die. This carries over from ME2 to ME3 -- including some romances if you choose to exploit that. Your name is Shepherd. You can play male or female. Female does offer some more interesting romances with the guys - especially with Garrus. I'm a guy & I played both genders.
At the end of ME3, the developers just quit, ran out of money, or were pressured to deliver (to market) so they just killed you off. Actually 5 possible endings but the net result is the same (you gone-funeral held). This insane ending caused literally YEARS of forum griping and was the cause of the fastest devalued game in history. Really, you will NOT be disappointed if you play this game (ME3) with a different mindset: Play the game for the fun of completing your quest (to unite the galaxy) -- not for the stupid ending.
Buy all the mods else you will be left clueless in some areas. One mod will have you rolling on the floor laughing. Now that I've played the series, I cannot imagine how much I would have missed. ME1 is okay. ME2 is arguably better than ME3 though ME3 has is vastly different with literally HUNDREDS of things to do. You're at war (robot invasion) at the start so make sure you play ME1 and ME2 first.