Why are the elites on the humans side in halo 3?

Go to Halopedian.com and look up the Great Schism. It explains in full detail. If you want the short version, here it is.

For thousands of years, the Prophets relied on the Elites for Protection and military strength. Then the Prophets betrayed the Elites by replacing them with the Brutes.

That's why when you play Halo 3: ODST you'll never see an Elite. Because the Elites were aligned with the UNSC during that conflict.

Also, the Elites and Brutes have had a long history of conflict. The next time you play Halo Reach, notice the groups that attack during campaign and firefight. You'll never see a group of Brutes and Elites together. Only groups of Elites or groups of Brutes.