http://www.equipped.org/soundoff.htm

National Park Service Safety Page

Mentions "whistle blasts, repeated 3 times, universal signal for help"

There must be a psychological reason for this, I cannot find it mentioned on the internet

Also
- On radiotelephone, MAYDAY, PAN PAN, SECURITE are repeated 3 times
- S.O.S. is 3 characters, each represented by 3 morse components (not sure what a dah or dit is called ...) (dit-dit-dit, dah-dah-dah, dit-dit-dit ) (... --- ...) (a dah is 3 times as long as a dit)
- With horns, 3 usually means "stop" or "reverse". Still, an anttention to something not normal
- Pavlof's experiment repeated the tone 3 time