I haven't played Halo Wars in a while so bear with me here.
I use to always use the main person, can't remember their name, but it's not the covenant. I think it was Forge?
I would just build a couple of the things that get you money, then a barracks, buy 3 spartans, a couple marines, get the upgrades for the spartans, then buy whatever allows you to make vehicles.
Then buy a lot of scorpions
But don't forget to upgrade your base, and the upgrade to make supplies faster.

This makes me wanna play Halo Wars again..

But if you happen to choose the covenant, make some Hunters and the other ones (not grunts, cant remember names) then you are going to want to build a scarab
While the scarab is in process of being made, your hunters can protect the base, you can make some aircraft to, I would suggest Vampires maybe with the health drain thing on.
If you want, after you have 1 scarab, buy the upgrade that allows you to have 40 people or whatever.
Then send your army (with scarab if you really want, but you risk loosing it.) while the other scarab is being made. but if you still have an army besides the scarab you gotta send the army to the enemy base and keep the scarab at your base, let the rest of the army die out, then you can make another scarab because a scarab is 20

Oh I almost forgot, buy turrets, and the base sheild.

Hope this helps you in some way I know it's a pretty long response

thats very helpful tactic but I need to make best 3V3 team with the best AIs on my team to beat 3 other AIs