You have 2 options in TCP - call or fold. When you fold, you lose your ante bet (duh). But sometimes you'll call because you don't want to just throw away that ante bet.

For instance, say you're playing regular Texas Holden (against other players). There's $1,000 in the pot. You figure out that your hand is most likely going to lose (10% chance to win), and the other player raises $5. Are you going to fold and have zero chance of winning? Or are you going to bet $5 to have a 10% chance of winning $1,000? Surely, you'd bet $5. Of course that $5 bet is at a disadvantage, but it let's you stay in the game for a chance to win.

In the same way, yet much less dramatic, you call(raise) in 3CP because it's better than just throwing away your ante.

When using a promo chip, I believe you raise(call) with actual $ if you have K,9,7 or better. If your hand is worse than K,9,7 you use a promo chip. You never fold if you have the ability to use a promo chip.