Who says there's a catch? Or . . ask them.
Actually, though, that model could work. Look at it this way: First, as a buyer there's typically no commission (to the buyer). So if you're the buyer and you're being asked to pay a 3% commission, there's income to the agent that you wouldn't get with the "traditional" arrangement of a commission when selling, but not buying.
As for the 0% commission selling, ask the agent. It's possible--I'm only guessing--that the listing agent might structure the sale the same way a FSBO would. Now, some FSBOs are willing to pay the buyer's agent commission. Others aren't. An agent might structure it like that: You pay no commission. However, the commission would have to be paid by the buyer. Be careful: That would serious limit the number of potential buyers who'd consider your home.
So: Ask an agent who's offering such a deal.
Hope that helps.