Ndtyhij

Yep, the critical commands are repeated 3 times. This insures there is ABSOLUTELY zero doubt in anyone's mind (especially on a big crew airplane) 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.

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 Herman 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 addition info on Mayday and Pan Pan.

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!”

the reason for the repetition of mayday mayday mayday is for receiver of the messages can hear the callings, if the first mayday calling is breaking, the second mayday calling maybe be heard, and totally sure the third mayday the messages needs to be convey along with the mayday

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

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.