i need help on level 3. i cant destroy whatever i have to destroy. i need a walk thru if possible.

so do i i dont understand that what the prevent HQ breach

Preventing a HQ breach means that you will need to kill all enemies on the map.

Refer to the strategy guide for one strategy or the way I do it is...

Team A
Rifleman
Demo
Support
Team B
Rifleman
Support
Team C
Sniper
Team B and C should take up defensive positions near the house that the Allies are guarding. Make sure that you can shoot towards the south (where you spawned at)
Take Team A past the house up to the cliffs near the bridge. You should be able to take out the patrol with little trouble. If you look over the bridge you should see members of the other patrol. Shoot them and you might attract the others as well. After a short time the tanks will approach. Wait until they are past the bridge and then you can shoot down at them.
Between your troops at the house and your troops on the cliff you should be able to eliminate most of the troops near the bridge. Enemies will also spawn at both ends of the RR tracks so look out for them.
To cleanup simply take the Teams and sweep both cliffs and down towards the end of the map (where the tanks came from).