Yes, but the person with the gamepad usually has a different role in multiplayer (e.g. in Nintendo Land on the mini-game Luigi's Ghost Mansion, the person with the gamepad is the ghost).
On games like New Super Mario Bros. U, it's either one player with one person using the gamepad or a wii remote, or multiplayer where the person with the gamepad taps on the screen to add blocks to help other players.
For Wii games, however, the gamepad shuts off and it cannot be used as a controller. So for wii games, you only have two controllers.
@Mattie I think you misunderstood his question. He wasn't asking how the gamepad is used, he was asking if one player on the gamepad can play along two other players who have wii remotes.