Need help with Ward mission

So, we got the new mission up. Ive finished them all but 1, and thats the ward mission.

I either have to have the finishing blow on a ward 3 times or i need to place 3 myself.

Well, ive been placing 3 multiply times, but as the noob i still am it doesnt succeed. There is the 146 second timer. as soon i place the 2'nd it only says i placed one.

What do i have to do to succeed? Much appriciated.