UPDATE: The below was written early in 2007. By mid-summer 2007, most of Halo 3's feature list was known; to date there is no mention of other Spartans, and the four-player cooperative feature uses the Chief, the Arbiter, and two Elite warriors as characters.

This is a perennial favorite question ever since the Halo backstory in Fall of Reach revealed that other Spartans did, in fact, survive.

Bungie so far has not announced anything specific regards to plot or gameplay features.

However, if you ask me, there are good reasons why so far Bungie has not included other Spartans in gameplay, and they still hold true. I seriously doubt this will be in Halo 3.

The Master Chief, as a Spartan, is somewhat of a super warrior-- more than a match for the best the Covenant have to offer. This is not because of his equipment or weapons, but because of his training and the intelligence brought to his avatar by the player.

For Halo 3's artificial intelligence routines to believably simulate another Spartan in the game would most likely be exceedingly difficult to balance. Either the Spartan character would be considered too weak to be a match for the Chief in ability, or else it would be too strong, and make the campaign mode too easy.