Capturing the convoy is easy, doing so while completing both secondary mission objectives is more challenging. At first, use your venoms to head to the northeast and focus fire on the tiberium tower, then flee from the anti-air immediately once it's dead. Then, send your four stealth tanks to the west to hit the sonic emitter. There is a hill there where it is safe to lie in wait for the convoy.
The convoys will try to keep moving when they come under attack. Focus fire on the elite APCs and tanks protecting them (the last has a mammoth which needs to go down quick!). Try to keep your saboteurs out of combat. When you see a safe time, send them to the southwest to capture the objective there.
Get all three convoys and keep the cargo trucks with your group. You can leave a venom and the tanks to protect the truck at the road while your stealth tanks go after the convoy. Just don't send only stealths after the convoy with the mammoth tank.
When you have all three, make your way to the rally point, moving slowly to avoid the scrin attacks. When you reach it, a countdown will begin. Use your cash you begin with to produce scorpion tanks to help absorb fire. Your elite stealth tanks will be the real stars here, as they bring aircraft to the ground. Watch for devastator warships (fires purple discs from a distance) that need to be taken down lest they slowly damage your army. Wait out the countdown and the mission is complete.