If the Ark is on Earth, there is the possibility that the Flood could be on it. As such, perhaps when the Covenant reach the Ark, the release the Flood by accident (again...they don't learn obviously). I still believe the Elites will eventually join the Humans when the Arbiter contacts them, so it would make it  three way. This would also support my John-activating-Halos theory. Gravemind doesn't have to get to Earth if the Flood are in the Ark.

Truth be told, there is no real answer here. I can merely speculate and weigh the probability, and right now I figure it to be a 50% chance the Flood are in the Ark, and maybe a 75% chance the Ark is on Earth. that makes it a...I think that would be a 37.5% chance the Flood being on Earth, but with Bungie you never know. The Ark is widely believed to be on Earth, and the Flood could very well be in it, so we cannot tell at this point.

do you think there will be a skirmish mode not only a campaign IM NOT A NOOB maybe a little

A lot of people don't seem to know the difference between noob and newb. Newbs are new and don't know, noobs are not new and refuse to learn. Noobs don't ask questions, so if you are asking a question then you are not a noob.

As for Halo 3, I have no idea...the multiplayer aspect of the game is not well know to me (I never had enough to buy Xbox Live), so I wouldn't be able to answer this question. Dmand, tis your turn.

there was an actual Sparatn I program that ONI had begun. The spartans were chosen for their skills and abilities. They recieved extensive training and minor augmentations, nothing compared to the 2 and 3. It is speculation that Sarge was a spartan 1 and further speculation that the borens syndrome that saved him from the flood was caused by the minor augmentation. of course that is mere speculation.

As for the multiplayer.. Halo 3 can be assunmed to have it usual game types. There will be the new weapons, Brutes new line of weaqpons. The redone MA5B which is now the MA5C some new features on the maps themselves is that the maps can fight u also, in a way, there are auto turrets and shield walls that provide a deadlier map. Also the new mongoose is going to make its appearance as the ATV. There is also the man cannon which can be used to launch yourself, grenades, ojects. the mongoose, and the worthog. find out more on bungie and  follow the article to the 1up site. there is tons of info there and it is from tours with bungie so at the moment it is all true with no speculation.

As for HW multiplayer i speculate that there will be skirmishes like C&C and possible like AOE seeing as how they made AOE. It is my opinion that halo would work much better as a C&C style, with a contrustion building a vehicle that goes around and build. barracks maybe even a flash clone device that could produce a unit that lasts for 20 min or something abd it cost a fraction of the original idk. of course this is all speculation b/c  ES has not released any info. hoped this awnsers ur question

Spartan-Is are mentioned in several places.

ilovebees is the major source for info on Spartan-Is, and though originally it was not canon, it is now embraced as canon.

The Halo Graphic Novel, when refering to Spartans, talks about "Ones" and "Twos". The Halo Graphic Novel is canon as well, though I think that there are some parts not excepted as canon...can't remember

The Flood makes a reference to the Spartan-Is as well.

The Flood page 16:

Like his fellow techs, Sam had heard of the program, though he'd never seen an actual Spartan in person. In order to deal with increasing civil turmoil the Colonial Military Administration had secretly launched Project ORION back in 2491. The purpose of the program was to develop supersoldiers, code-named "Spartans," who would receive special training and physical augmentations. The initial effort was successful, and in 2517 a new group of Spartans, the II-series, had been selected as the next generation of supersoldier.

Rotaretilbo:

Spartan-Is are mentioned in several places.

ilovebees is the major source for info on Spartan-Is, and though originally it was not canon, it is now embraced as canon.

The Halo Graphic Novel, when refering to Spartans, talks about "Ones" and "Twos". The Halo Graphic Novel is canon as well, though I think that there are some parts not excepted as canon...can't remember

The Flood makes a reference to the Spartan-Is as well.

The Flood page 16:

Like his fellow techs, Sam had heard of the program, though he'd never seen an actual Spartan in person. In order to deal with increasing civil turmoil the Colonial Military Administration had secretly launched Project ORION back in 2491. The purpose of the program was to develop supersoldiers, code-named "Spartans," who would receive special training and physical augmentations. The initial effort was successful, and in 2517 a new group of Spartans, the II-series, had been selected as the next generation of supersoldier.

Why would they have a story that is Part cannon and part NOT cannon, i dont think that makes too much sence............. ( i am a Horrible Speller)