how to create a script for a falling platform?

I am wondering how to create a script for a platform that will start falling in about 3 seconds after contact with the player.

Put a collider on the platform, set it to Is Trigger, then in a script on that object, OnTriggerEnter handler, have it wait 3 seconds then play the falling animation or however you want it to fall (have graivity off normally then turn it on, script it to move down in Y, whatever). If you want to have it fall only if the player stands on it for 3 seconds, use OnTriggerStay and keep track of time there.

Start a coroutine upon collision. Before you start the platform falling in the coroutine, have it do a:

yield return new WaitForSeconds(3.0f);

you could even write this into a seperate script that is attached to each platform.