Who would command the Enterprise D, if Captain Picard, Commander Riker, and Commander Data were off the ship?
Of the main cast, apart from the three ranking officers mentioned, we also know of several instances of such cases:
- In TNG 'Descent', Beverly Crusher takes command
- In 'The Arsenal of Freedom', Geordi La Forge takes command when all the ranking officers are off the ship
More importantly though, it depends when you are referring to. At various stages, several commanding officers were on the Enterprise-D. This list assumes that the Captain, First Officer and Second Officer (whoever they were at the time) were off the ship
- Lieutenant Commander Sarah MacDougal
- Lieutenant Commander Argyle
- Lieutenant Logan
- Lieutenant Commander Leland T. Lynch
- Lieutenant / Lieutenant Commander Geordi La Forge
Chief Medical Officers
- Commander Beverly Crusher
- Commander Katherine Pulaski
Additionally, once Troi passed her Command test and achieved the full rank of Commander, she too could command the Enterprise.
I would say Commander Beverly Crusher, as she is a fully certified bridge officer and has the rank of commander. She took command of the enterprise on numerous occasions (see her Memory Alpha article)
Presumably,as well as Picard,Riker etc, the ship could also be commanded by Tasha, Worf, Pulaski or whoever the captain chose-even down to Ro or Wesley!
By rank alone, after Picard, Riker, and Data, the next in command would be Deanna Troi. Troy holds the rank of Commander and during the time this character was on Enterprise-D/E, she had her own watch (4th watch).
Well I would suppose Geordi, as that was what happened in the Arsenal of Freedom, but he isn't the highest-ranking officer there, so I suppose generally it's whoever Picard/Riker/Data tells to have the bridge before they leave.