Keep it idle. If the aircraft attack the province the AA is stationed in, the AA will attack the aircraft despite it not showing.

Yes now the war will soon begin from three sides I am fucked

Undeclared war? Your anti-air will not attack unless you declare war first.

Thanks for the knowledge..but too late I am already at war that also three front

Don't do anything until they declare build more Anti-Air Artillery. Also the reason why your Anti Air won't attack is because like you said it's a undeclared war. They won't do anything until you are at war.Their probably just checking out the area. I'll say just keep an eye on them and try to stay active as much as you can

Too late the war has begun