Mass effect 3 sounds like it is an amazing game, I just might get it tomorrow since I happen to have the money for it now and a few pay checks on the way.
I'll give a review (without spoilers) probably tomorrow. maybe later.

As for the first two games they are DEFINATLY worth getting. Now the first one had a few problems that made it a little irritating but otherwise an amazing game like the second one, but there is a 15 minute DLC for ME2 that allows you to make all the decisions from the first one so you don't have to go through that game as well as the second to save you some time. It is pretty awesome to see how your decisions affect the next game and how much can change.
it may be sort of "open world" but it is fairly straight forward, there are secondary planets to find and complete extra missions but they point you straight to the objective, Mass effect games are not as free roaming as elder scrolls with climbing mountains and discovering a cavern on top. Still feels pretty open though.

Also, here are a few examples why you should get them.

http://www.youtube.com/watch?v=sy-eRfupYbA

http://www.youtube.com/watch?v=-PjT.....feature=fvwrel

yeah,it does look like real amazing game, but like you said, i feel i have to play the first two games to know hat going on in the 3rd nd i really don't have the time for all that.