[mikeash]

Back in the bad old days of pre-UNIX Macs, it was a common troubleshooting step to reset your PRAM. This is battery backed up RAM that holds some basic settings, and if it got corrupted somehow it could cause weird problems. You'd reboot while holding down command, option, P, and R, then wait for the boot chime to sound a second time indicating that it had been reset, then release the keys and boot normally.

Somehow this advice got mutated so that you'd keep holding the keys until you heard two boot chimes (thus resetting the stuff twice). And then it started to grow. Three was common. Some people would advise more. I'm pretty sure that doing it more than once never helped anything, but there we are.

(The cmd-opt-P-R sequence still works on modern Macs and I actually used it to resurrect a machine that wouldn't start up just a month ago, but it's far less frequently needed now.)

[celebril]

It's the same with the battery stats resetting and the Dalvik cache wiping these days in Android land. You do it three+ times.

Or the "Repair permissions" thing in OS X. You do it several times as well.

It's like whenever there's this one-step fix thing that a system utility does, the Common Man will interpret it as needing to repeat 3+ times in order for it to be effective.