Since you started with a heat lamp you will need to slowly ween them off of it. They typically need to adjust to the normal outside temps and be mostly feathered. Starting without a heat lamp and using other methods for heat generally causes them to be a little hardier and feather out faster.
Raise the heat lamp a little every few days so as the lower the temp in the brooder by 5 degrees or so a week until you reach your normal temps which is usually around 65-70.
At 3 weeks of age the temp inside the brooder should be 80-85 , once the chicks are fully feathered and provided your outside temps are mild , they can go outside . I start taking mine out during the day for short periods from week 2 and gradually increase the exposure time . This link has a temperature chart that you can use as a guide.