THIS IS A SOLUTION FOR 3 STAR MEDIC, COVERT OPS AND ENGINEER.
Start a private match with you and a friend. Set it up a 30 min objective on Slipgate, 4 players, bots on hard skill and finishing tasks. with both players on GDF.
Once you have started the match both players switch to medic and throw down a supply crate. Once you have thrown down the crate start shooting at anything (I like shooting the 4 wheelers) until the crates arrive. Once they have arrived stand next to crate that is NOT yours and keep firing. the crate will keep refilling your ammo as well as give the other person XP keep throwing down crates until you both have 3 stars as medic (it should take about 5 - 6 mins and 3 and a half crates)
Once you've got that both respawn as a Covert ops and take the vehicles over to the slipgate (watch out they more than like have a mine on the slipgate controls and a Anti Vehicle Turret up by now) now take turns hacking the gate to 95% and than letting it drop to 50% and than hack it to 95% and let it drop. repeat as needed until both players have 3 stars (when the other player was hacking I liked to either respawn as a Soldier and blow up the 3 gates, or I would stay as a covert ops and run to the AVT and disable it giving me even more XP to make thinks go faster)
Once you have got that done the player that is not the host (host is unable to switch teams) switch to the strogg oppressor (engineer) and go thru the slipgate and put down a Anti Vehicle Turret next to the place that the MCP needs to be taken to. The host will respawn as a Engineer and take the MCP thru the slipgate and up to the end where the Anti Vehicle Turret is and it will start killing it. Host keep repairing the MCP until you have 3 stars. The other player is the lucky one cause he doesn't have to do anything but make sure people don't kill the host while he is repairing the MCP (do this by team killing the 1 bot that is one your team) and also make sure the bot on the GDF doesn't take the MCP thru to the end. You should both get the 3 stars with 30 seconds to a minute of each other.
If I'm not mistaken depending on how fast you are you should have 5 - 8 minutes left in which you can switch to a GDF soldier and complete the final objective to end the game. (If you blew up the 3 gates in the Covert Ops stage you should be a little over 1 star as a soldier so if you blow up the final objective as well you can 2 star the soldier as well)
Hope this method helps you out as much as it helped me!! Good luck and if you have and questions drop me a line and I can either explain it to ya or if I'm feeling really Generous and have time I can run thru a round with you.