How option premium will be calculated on the day of exipry, by ATP OR LTP

why the premium will falls after 3.00 pm on the expiry day !

The option premium on expiry is always calculated by its intrinsic value.

For CE -

Max [Spot - Strike, 0]

For PE -

Max [Strike - Spot, 0]

If you are new to options or would like to learn more, then I’d suggest you start here -