Star Trek

Spock (first officer) is second in command. But--who is third?

That is to say, if Kirk and Spock happened to be...say...incapacitated or, somehow, away from the Enterprise on some kind of mission, or something like that, who would be in charge? Who would have the authority to "command" while the other two are not present? Who would be that "you're in charge until further notice" person?

Bones? Scotty? Uhura? A random "yellow-shirt" guy in the command division?

I'm sorry. But this has just been bugging at me for quite a while.

I am fairly certain that Scotty was the second officer since he was the Chief Engineer.

Hm. Reasonable. But he would be too busy keeping the ship running, wouldn't he?

Pretty sure it was Sulu. I know he was third in line in the Mirror Universe.

I bet it is Scotty. In one episode, I think it was "Let it be your last battlefield" he was the third person giving the computer the authorisation sequence to destroy the Enterprise. Therefore in my opinion Scotty ist the third most important person on the ship and in command when Kirk or Spock are absent.

Who Kirk left in charge seemed to vary with the situation, but based on the number of times Scotty was left in command, I believe that he was next in line after Spock, followed by Sulu. There were multiple episodes where Kirk and Spock left the ship and kirk put Scotty in charge, but whenever he had to leave the bridge and run to engineering during that time, Scotty would put Sulu in command.

Ack! Sulu?! Well, my mind is certainly blown for the day. 

Maybe it's because I'm not fond of the character. An Enterprise with Sulu in command...

Even Mr. Kyle seems better suited.

'In Universe' it went: Kirk, Spock, Scotty. Then, maybe Sulu or the Assistant Chief Engineer, DeSalle (or LaSalle, I can never recall?) in 'Catspaw.'

In the actual episodes: I think it went by how much money they had. If they could afford Jimmy Doohan, or a lesser-paid extra.

Most of the time if Scotty wasn't left in command it was either because he was part of the landing party or he was working in engineering, either trying to keep the ship together or on some project the captain assigned him to. And, of course, George Takei missed a lot of episodes while he was filming The Green Berets with John Wayne, so there were a lot of opportunities for command that Sulu missed out on while that was going on. And in Catspaw, Sulu was captive on the planet, which is why Desalle was left in charge.

I'm pretty sure Scotty was next in line after Spock. 

If they run the Enterprise like they do a Navy ship, then after Spock would be the Chief Engineer, Montgomery Scott.

Having watched for decades, I'm absolutely sure Scott is, his rank being Lt. Commander.  Next comes Lt. Sulu, by vitue of his helm training alone - anyone who has handled the helm is up for it - then Lt. Uhura, who has subbed at the helm. 

Scotty usually. He sometimes had a large command role, as in the episode "Bread and Circuses."