so I kind of want to get this, but I also haven't played the original Halo 3. Am I gonna be totally lost here or is Halo ODST pretty much a side story?

Avoid all of the Halos and play a better game... Half-Life 2! Weeeee!

Its a side story that takes place during Halo 2, but you don't have to have played Halo 2 either to know whats going on.

You don't have to have played Halo 2 or 3 to understand this side story.

Its pretty self-contained. 
  
Other than being familiar with the weapons, vehicles, and enemies, theres no real tie-in

" Avoid all of the Halos and play a better game... Half-Life 2! Weeeee! "

ODST has almost no relation to Halo 3- it actually expounds on a small plot point from Halo 2 (showing what happens to New Mombasa in the aftermath of the Slipspace Event). Completely independent and you should have no problem following what's going on.

" ODST has almost no relation to Halo 3- it actually expounds on a small plot point from Halo 2 (showing what happens to New Mombasa in the aftermath of the Slipspace Event). Completely independent and you should have no problem following what's going on. "