Lazybones said:I've played all 3, and while there is a story connecting them all, it's my opinion that it isn't a necessary prerequisite to enjoying the action. Basically you run around and shoot things, occasionally taking a break to drive around and shoot things, or fly around and shoot things.
I also played Halo 1 on PC, then Halo 2 on backwards compatibility on my 360, then Halo 3. I got H2 for about $20 used via an Amazon 3rd party seller.