A third stage with the current Starship design would have to be under 100 tonnes and the payload would be small. It would have to do all the work of injecting into a transfer orbit, landing on Mars, and returning. It’s hard to imagine such a small craft getting all the way to Mars, with a crew, and returning them.
Starship is just not a good design for pushing a third stage into a transfer orbit by itself. It’s totally dependent on the idea of in-orbit refuelling and refuelling on Mars. Once you refuel it, the game changes completely.
It’s also not a workable solution for landing on the Moon without refuelling for the same reasons. In some ways the Moon is more problematic because you can’t manufacture methalox on the Moon.
Starship is just not a good design for pushing a third stage into a transfer orbit by itself. It’s totally dependent on the idea of in-orbit refuelling and refuelling on Mars. Once you refuel it, the game changes completely.
It’s also not a workable solution for landing on the Moon without refuelling for the same reasons. In some ways the Moon is more problematic because you can’t manufacture methalox on the Moon.