I recently played a 3 star campaign and it played out like this, all in all it took about 3 days at about 4 hours a day to complete.
I started with myself and 1 normal AI and 1 hard AI on my team, I was TEC, and my team was Advent and Vassari.
We were playing against 3 hard AI, 2 normal AI and 1 easy AI.
It was a 3 star system so I ended up with my 2 teammates in the other 2 systems.
Well it didn't start out so well, I wound up starting out with 2 of the hard AI as my neighbors (ouch) but I quickly struck up a cease fire deal with one and swarmed the other all the while. After I got my only hostile direct neighbor I spent time building and researching (didn't increase fleet size to quickly) and shored up my defense.
My planatery defense worked out like this,
4 repair platforms in tight formation, like sitting on top of each other.
4 hanager defense platforms in tight formation next to the repair platfroms, I mixed it up with a couple of bombers but it was mostly fighters.
And the rest was turret defense, but they were very tightly packed in and around the hangers and repair platfroms as well.
Not flawless, but you can toss a handfull of LRM friggets or carriers or maybe a captial ship and just have them hold thier ground. This setup held my main defense (gateway) world the entire match.
After I established myself in research I then built up my fleet and started working on the politics, I even wound up turning on my own team mates (I don't know why but they had allready built trade alliances with him) in order to build an alliance long enough to assault the other hard AI.
After that part, (it was the hard part) I aquired enough worlds to get enough income to finance a novalith cannon on almost every world I own, I fixed my allaince with my team and the game fell into a partern of.
1. Get a cease fire with almost everyone.
2. Nuke the unlucky person who didn't agree with my terms.
3. Take his planets.
4. Rinse and repeat.