I've done this before on the Original xbox version im still working on the 360 version but I have some tips that may help
1. Stay alive! Any wasted men makes your stars drop, don't be a hero and make sure your teammates(the AI) stay alive as well.
2. Be accurate! Every shot counts! The better your accuracy the more stars you get. Make sure you only shoot when necessary!
3. Be quick! Most missions have timers to designate when you need to beat it. All Missions have a par time to beat! You will most likely have to repeat missions multiple times to get it. Since the levels are "staged" it doesn't take long to figure out where everything is. Practice knowing the level before you really go for the perfect mission!
4. Dont just go for particular stats. You have to get a perfect mission to attain all stars you cant just go for some in one run then more in another. It all has to be done at once.
5. Keep your multiplier up! This is the most important as score is what gives you the majority of stars! If your multiplier gets up higher your soldier gets stronger and more deadly the better it gets. Just remember don't go crazy with power you gotta remember accuracy!! Dieing also effects your score drastically! If you think you are gonna die, do a hotswap with an AI to keep yourself from losing points.
As for the 3 Star missions (extra missions) Most of them are really simple and only have one objective to pass. Some are racing, Shooting, Point to Point, etc etc. Just Remember learn the map and what you have to do and most likely if you pa attention to the staged actions you can have a perfect in 2 or 3 goes.
I hope this helps as nothing else can really. If you wanna watch youtube videos for missions they are available but most are vague. Just make sure to not go rambo unless necessary! Try to be a medic if possible and you can just auto inject yourself. Auto injecting teammates also really gets you a decent amount of points!
P.S. Even if it keeps you from dieing dont always go snipes when possible! It'll usually screw your AI over.