The 3 second rule for popcorn is a cooking guideline for popcorn, in which you stop heating the popcorn when the time between the pops stretches to about 2 seconds to 3 seconds, which prevents your popcorn from burning.
Instead of relying on a timer when popping popcorn, you listen to the popping rate of the popcorn as it cooks to know when to remove the popcorn from the heat or microwave.
To use the 3 second rule when popping popcorn, listen carefully and pay close attention to the sound of the popping of the popcorn.
Then wait for the pause.
Once the popping has slowed to a consistent pause of around 2 seconds to 3 seconds between each pop, then it's time to stop.
Immediately remove the popcorn from the source of heat or stop the microwave, depending on how you're popping the popcorn.
Don't way for every kernel of popcorn to pop as the last few popcorn kernels may still be unpopped, but if you continue to cook the popcorn, then the rest of the popcorn will burn.
Also if cooking microwave popcorn, be careful when opening the bag as it can cause steam to escape and burn you.