When the 360 halo games go backwards compatible i assume the mcc will be pretty much dead and i will need to re purchuse some of the older tilles. Halo 4 and odst make sense to rebuy. My main question is should i repurchuse 3? Because 3 odst comes with h3 multi, and i have the campaign on mcc, so is there really a reason to get halo 3?
It’s all up to your tastes, buddy. Knock yourself out! 
Not sure why would you want H4, but the MCC version will likely be more popular since it’s the better version of it. For the money you’d spend on H4, use it for H3 instead and get ODST as well.
I would get Halo 3 because the Halo 3 port for MCC is really terrible. The 360 version of Halo 3 is much better and also you would be getting more with 3 than you would with ODST or Halo 4 or even Halo CEA. You would be just getting Halo CEA for the campaign because the Anniversary map pack for Halo Reach is free. Halo 3 ODST you would get for Campaign and Firefight. Halo 4 you would be getting Campaign, Multiplayer, Forge, and Spartan Ops. However Halo 4 will most likely have the lowest population out of the BC Halo games. So in my opinion you should get Halo 3. You would be getting Campaign, Multiplayer, and Forge. But again I guarantee you that Halo 3 will have the highest population out of the BC Halo games. But at the end of the day the choice is yours!