There are 3 main ways this could have happened.
Glitches: The main Glitch that affects shiny rates in the Cute Charm glitch. This page over here explains the glitch, but yeah, if you had a Pokemon with cute charm as your lead, this is probably why you encountered 3 shiny Gligar. Also this, RNG manipulation.
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Luck: The 3 Shiny Gligar could also just be plain luck, although there is a very slim chance this actually happened.
You have a fake copy/reproduction: If you have a reproduction, there is a chance the person who made the reproduction, accidentally tinkered with some of the code, causing some of these rare occurences.
I hope this helps
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