It takes 3 of the same card to make a 2*, and 3 of the 2* to make a 3*-- try to avoid spending paws in the early rounds, which gets easier as you get more experience with which cards are "good" (krane, the new chinchillas, some players like stacking cards that share pack mentality because they will buff each other. Avoid the spotty cows, they often skip their attack turns) -- and focus on stacking up extra paws when you can, so you have more paws to waste refreshing the shop to get more of your desired cards in your deck during the middle and end.
I second their LW counterparts & the bony salamanders absolutely trouncing in the arena, great additions. Hard to go wrong with 'em, CT, I suggest pickin' 'em up in your games every time you can!