Halo's campaign sum up
Halo 3's story centers on the interstellar war between 26th century humanity, led by the United Nations Space Command, and a collection of alien races known as the Covenant. The player assumes the role of the Master Chief, a cybernetically enhanced supersoldier, as he wages war in defense of humanity, assisted by human Marines as well as an allied alien race called Elites, which is led by Rtas'Vadum.
Did you watch all the cutscenes if you did you would understand what times each game in the series is set at for example.
Reach - Halo 1 - Halo 2 - Halo 3 - ODST.
Yes that's right reach was set before halo 1.
But yeah , did you watch all the cutscenes in halo 3's campaign.