I think the point of having a crew that can take care of combat is to make sure that the tank can fight as an independent unit.maybe that is seeing it from a WWII point of view, but what if in the future the communication systems are so advanced and reliable that the commander is someone from a base giving orders to the gunner and the driver inside the tank? instead of eliminating the gunner eliminate the commander, wouldn't that be possible?
In a gunner + driver + remote commander scenario, one the communication is disrupted, the tank is screwed.
If communication can be 100% secured(quantum entanglement communication?), manless tank technology is achievable today. There wouldn't be any point to put the drive or gunner in a tank to risk his life. (Other than for field repairs)