It's a good game. It helps to understand a little better the story if you played the previous games, but you can still enjoy it without playing them.
If you like the game, I suggest that you buy 1 & 2.
I saw them in the bargain bin for 9,95€ each so it's a really good price.
Hey just wanted to let you know I took your advice after playing a few hours of ME3, I went ahead and bought 1 & 2 Gamefly has them on sale today for digital download $6 each.