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!