If they are in the house, you should start turning the lamp off for increasingly longer periods of time. What are you using for a bulb, and what size is their brooder? How many chicks? By the time they are 3 weeks old, they should be off heat completely in the house. depending on your set up, if you have them in a room that can be closed off, you could open the window to chill the room to help acclimate them to outdoor temps.
My chicks are brooded with MHP in a grow out coop. By the time they are 4 - 5 weeks old they have weaned themselves off heat completely, with night time temps dipping into 30's, day time temps 50 - 70.