For the right offer I wouldn't mind seeing the Kings give up any of them. But I'd be most reluctant to trade Casspi.
Donte would be the easiest to part with. He seems like a great teammate and he has great size but he continues to show little to no development. I don't see any real fire with him.
Garcia is a nice glue guy. Nice backup at SG/SF who gives good effort and direction on the defensive end, some versatility and shooting on offense and who serves a mentor role with this young team. I'd be more reluctant to give him up. That said, he has a good sized contract, especially for a bench player, he's likely maxed out his potential and will turn 31 this year.
Casspi to me has a good combinaton of youth, potential, passion and a true position. Not to mention a very reasonable rookie contract. At best he entrenches himself as the starter at SF as this team makes strides towards being a playoff contender. At worst he's a good energy guy off the bench.