Finally got 3 Stars on missions 12. Here is it!
1. Don't let mortars near your troops in the top left base. Tanks need to take out the mortar turret. You might lose one tank to the rocket.
2. Save around 25 troops for the bottom base. Make sure to take out rocket turret before your tanks get there. This very important.
3. Send 2 troops to right and bottom base to take out shield before your main army gets there. This is not completely necessary but will help with AI in the right base battle
Honestly, a bit of this mission is luck in the path your army decides to take. It will determine whether they get there too early to be killed or just in time to help out.