>>46048
Well besides the aforementioned Trinity, thee appears in quite a various forms all over.
Remaining at religion Hinduism has three main gods (Trimurti): Brahma, Vishnu, and Shiva. Or some people believes the Norse gods are just three (Odin, Frigg, Thor or Baldr not sure which), and the rest are their aspects (although this is might be just a Christian influence). And then in Christianity it also appears in different forms, for example the resurrection happens on the third day, or the devil tempts Jesus three times, or Peter denies Jesus three times.
In folk tales appears in Cinderella for example (Grimm version), she gets two step-sisters, making them three, and the king's festival takes three days.
In literary works, like at Homer: Paris have to pick one goddess out of three as the most beautiful.
In arts it also appears, like the motif of triskelion.
Just from the top of my heda. I'm sure we could find many more.