At the very beginning of your turn, before any attack, you can decide to play one (or more) tris to get additional tanks to deploy.
Here are the valid combinations of 3 cards:
3 horses = 8 tanks
3 knights = 8 tanks
3 cannons = 8 tanks
Horse + knight + cannon = 10 tanks
Jolly + 2 cards of the same kind = 12 tanks
One more thing: for each territory you own among the tris cards, you get +2 tanks!