Do you have other chickens in the coop right now? If yes, don't add a heat lamp. The other chickens will be too hot. What's the temperature like in the coop at night? Depending on your coop size, design and placement, it won't necessarily reach the ambient outdoor temperature at night, or if it does it may take it hours to cool down to 60 from 90. Coops can heat up and hold heat. So the inside of the coop may actually stay warm enough at night, and not need extra heat for the chicks. Get a thermometer in there and measure what the temperature is at night, and if it even gets as cold as the outside temperature.
That's why flock raiser is the best. It doesn't contain added calcium, and works for all ages of chickens. It's 20% protein which is good for both growing chicks, and laying hens. Have those chicks had medicated starter yet? If that's what they've been eating so far, and they're 3 weeks old, then they don't need it anymore and you can put the whole flock on flock raiser, with calcium on the side. If they haven't had any medicated feed, you can put the whole flock on medicated starter for a few weeks (or until they finish the bag), then switch everybody to flock raiser. That way you avoid the potential issues of chicks eating too much calcium.