420,
I see. That sounds like a good plan. In fact that's exactly what I did. Had mine in 8" diameter x 10" deep pots until they showed sex. Getting rid of the males gave me the room to upcan my remaining females into 3 gallon pots.
If you're only 3 weeks into total growth (and I'm assuming you're growing from seed) and have roots growing out the bottom of that sized container well, you're definitely doing something right to get a beast of a root system like that this quick. In fact I think I'm a little jealous. How about telling us a little more about how you're growing (light cycle, soil, nutes, etc.).
I'd suggest you read a thread on here called "Gardening Tips and Tweaks by Uncle Ben". He's got some good advice on upcanning. One thing that comes to mind that he recommends (since it sounds like you have quite the dense compacted root system) is to make some vertical cuts 1/2" deep (with a razor knife) into the root ball when upcanning. This will help promote new roots that will grow out into the surrounding new soil in the larger pot.
Jack