How should I know the completion of a three year to sell an ELSS fund?

The 3 year period ends from the date of transaction. So if you are into an SIP then the 3 year will apply to each purchase in the SIP.

People who want to switch to direct plan ELSS funds and avoid commission are stuck with that purchase and have to wait for 3 years till the date.

In the Maxiomwealth app there is a convenient reminder when the 3 year period is about to get completed.