With the latest update, you can earn stars on each gametype and map by hitting a score requirement for the gametype.
For most gametypes, 3 stars can actually be quite the challenge.
I wouldn't recommend Marked for Death as you need 70% of the score just to get it.
The 2 easiest gametypes (in my mind) are
Last Titan Standing - In a full 6v6 game, the winning team needs a minimum of 24 titan kills. So if you can focus on making sure you get the killing blow it shouldn't be too bad (8 kills for 3 stars).
Hardpoint - The stars are based off assault score, so if you end up in a game where no one is defending and instead just running around grabbing points, this isn't too bad to get.
You need 2500 assault score for 3 stars. You get 250 assault score for a capture if you were the first person in the base, and 100 points if you weren't. I believe any kills while taking the point also give assault score.
If you aren't too good at the versus, then you could get a team together for Frontier Defense, but you need to do fairly well to get 3 stars. I was able to get 3 on colony by 28 points. You're going to want a full team of 4 that you can communicate with for this though. What we did was any round that had only mortars for the titans we'd have 3 on offense and one on defense. If it had mortars and nukes then we'd split 2 and 2.
Just play whatever gametype you're comfortable with if you don't want to do these, I just feel that the top 2 gametypes are the most lenient and Frontier Defense is a good choice if you aren't good at pvp.
Update - I was able to get this very easily on Frontier Defense on Overlook. Essentially every wave is just Nuke Titans and Suicide Spectres. (At least this is how it was for us, I'm not sure if there's any randomness to it.) So as long as you watch the map and block the paths this is a very easy 3 star. We got 3 stars by over 100 points of what was required, the tower didn't even get damaged.