The best replacement for halo 3 is call of duty 4.

The best alternatives to halo 3 are rated as following:
★★★★★  call of duty 4
★★★★★  halo reach
★★★★  halo 3 odst
★★★★  gears of war
★★★  odst
★★★  halo 2
★★  halo 4
★★  reach
★★  killzone 2
★★  bioshock
★★  cod4
★  cod 4
★  halo odst
★  halo 4 graphics
★  modern warfare 2
★  gta 4
★  gears of war 2
★  mgs4
★  halo reach multiplayer
  half life 2
  halo reach graphics
  metal gear solid 4
  halo 4 sales
  metroid prime 3
  mw2
  crysis
  resistance fall of man
  halo anniversary
  halo wars
  gow
  gears of war 3
  call of duty
  killzone
  fallout 3

Your alternative for halo 3 is not listed? Suggest!
Speaking about halo 3, one probably shouldn't forget about crackdown.

Other search terms related to halo 3 and their ratings are:
★★★★★ crackdown
★★★★ xbox 360
★★★ xbox
★★ master chief helmet
★ halo odst
★ odst
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halo 3
call of duty 4
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