You could consider just calling in hopes of encouraing the other player to join, but he'll fold if he has any idea what's going on. It's fairly obvious you're slow-playing.

The middle position player is already becoming fairly committed, and will be completely so if he bets the river, so raising isn't necessary to win that player's money, but calling may help you win the other player's, so perhaps calling is best, as there's some chance he won't be smart enough to figure out what you're doing.