What do the other 2 crew members do? Can it be operated without them?

Additional crew members will run the turret, navigation, scanners, repair team or whatever the captain assigns them through the multi-crew seat system. One thing to remember is that we build our ships as they should 'work' in the game, even if some of that isn't as exciting for gameplay. In the Star Citizen universe, it makes sense for larger transports to have crews that 'trade off' the pilot seat during long-duration runs.