Yes because if all the map packs, even the ones you don't own, on Halo 3 will be on the multiplayer disk that comes with ODST, unless you just buy the ODST game and not the multiplayer disk witch I don't think is possible.
if your talking about which is better i think halo 3 because there is no multiplayer in halo 3 odst only firefight wich gets boring halo 3 ODST is mainly solo fighting (lone wolf stuff) but halo 3...
You play the roles of elite human United Nations Space Command soldiers known as Orbital Drop Shock Troopers (ODSTs) during the events of Halo 2 and Halo 3.
It really makes no difference. ODST's storyline takes place during that of Halo 3, but does not interfere or spoil anything from Halo 3. So, playing one before the other makes no difference.
halo 3 odst is basically about an orbital drop shock trooper who crash lands on earth. I don't know what the main missions are but I know that you don't play as master chief. try and go on wikipedia...