When nothing has happened with your mycelium box for three weeks, we suggest you use the "Cold Shock" technique to kickstart the development of the mushrooms.
Here is how you do this:
Take the full grow kit as it is now (inside the closed grow bag) and put it in the refrigerator (2 - 7°C) for one night (8-12 hours). After this time, place the grow kit back on the original warm spot. Wait a few days till a week before the pins strart to show.