Yep, the critical commands are repeated 3 times.
This ensures there is ABSOLUTELY zero doubt in anyone's mind (especially on a big crew airplane) of what needs to be done in a critical situation.
It also standardizes these criticalities across different aircraft and aircrew cultures.
"Bail out, bail out, bail out" "Eject, eject, eject" "Abort abort abort" " Pan-pan, pan-pan, pan-pan ."
I was 27 years a USAF pilot, and this is how the training has worked for over 50 years.
I only saw these terms used 2-3 times, but it certainly gets your attention and amps up the sense of urgency.
A little history:
back in the day of very poor radio communications, it was necessary to repeat to "get someone's attention" or in the event
a single "mayday" didn't come across when the transmit button was pressed.
aviation.stackexchange.com/users/36591/Scotty
Procedure calls for the mayday distress signal to be said three times in a row so that it won't be mistaken for another word or phrase that sounds similar under noisy conditions.
The use of Mayday dates back to 1923 when it was first used because it sounded like the French word m'aider, which means “Help me."
In those early days of radio it was necessary to repeat things sometimes because of interference on the frequency from various potential sources.
The "rule of three" is rooted in research conducted in 1890 by Hermann Ebbinghaus, a German psychologist.
Ebbinghuas studied how many rehearsals were necessary for his test subjects to memorize a list of nonsense syllables.
He came up with three as the optimal number, and that became a rule of thumb in many other things, such as advertising.
Here's a cool video that adds information on Mayday and Pan Pan .
aviation.stackexchange.com/users/36621/Clint Kearns
There are no instances in normal conversation where the same word is repeated three times consecutively.
In order to prevent a critical command or order from being issued or heard accidentally, a command is given three times in order to verify that it is being given intentionally.
Going to the moon?
“Launch!
Launch!
Launch!”
aviation.stackexchange.com/users/36589/Paul Willett
I assume it's for redundancy.
Assuming the voice signal is very noisy, the listener might hear two different things, the first and second time.
The third repetition can then be used to decide which of the two versions heard is more likely to be the correct one.
Majority voting with three signals is very common in redundant systems.
In computing it is called TMR (triple modular redundancy).
https://en.wikipedia.org/wiki/Triple_modular_redundancy
aviation.stackexchange.com/users/36605/user1323995
Because human brains are slow and easily distracted?
The first time you heard it - you started listening.
The second time you heard it - you started listening properly, because you know it's important
The third time confirmed you heard what you thought you heard?
This is just my unresearched perception of what's going on, and why we naturally settled on saying thing 3 times when it's imperative that it's heard properly.
aviation.stackexchange.com/users/20323/djsmiley2kStaysInside
Repetition is a key characteristic of communication in the control tower, cockpit, and control room.  Some phrases, like "Mayday" get repeated. The speaker says the same thing three times. We know this is for redundancy.   Why exactly  three times ?    Why not twice or four times? Is there research suggesting three is the most effective number, or is there a historical reason for the convention?
Why do we say certain things three times (e.g., "Mayday, Mayday, Mayday"), rather than two or four?
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<safety><radio-communications>
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aviation.stackexchange.com/users/27883/Mark Jones Jr.
Repeating a command three times is for redudency. If a command was misheard the first or second time, the third acts as confirmation. It is also a standardised practice across airlines and aircrew cultures. The 3 times repetition works in order to eliminate any possible doubt. It is also  for redundancy and reassurance. Another practical reason was for certainty due to the quality of radio transmission back then. And finally, it works as a manner of intercultural standardization.
In order to eliminate any possible doubt. It was necessary for certainty due to the quality of radio transmission back then. It is for redundancy and reassurance. It works as a manner of intercultural standardization.