Im not calling you a liar, saying you're mistaken or anything else in any way, but I've never heard of the 3 seconds before a pin. At any level. It has to be a modified rule in your area/with the group you were involved with. Nothing wrong with it at all if it was used for beginners, but don't base all wrestling on that rule.
Well that's how it was done in my summer camp and that's how the coach explained it. Maybe it's done differently in other camps, schools, or wrestling organizations but that was my experience.