You are in the defence, so the enemy is making a deliberate assault against you. That is the mission where most arty gets assigned.
- the AI does not use artillery as intelligently as a human opponent would
- As a defence, get an experienced core arty battery or two, with range >= the usual enemy arty. To get experience, best buy the battery in the initial game of a campaign. Just leave them idle, and they will get some CB fires in.
- As to AI AAA, see the existing recent thread on this.
http://forum.shrapnelgames.com/showthread.php?t=47909
Cheers
Andy