well, imo, you could repot at this point, it really wouldn't hurt. It also wouldn't hurt to veg it out a little bit longer as I doubt they are root bound yet, but it they are probably getting close. But for three weeks old, they honestly ought to be bigger and more established by now. My 3 wk girls are usually repotted and topped 1st time at around this point. *assuming photo, if auto repot ASAP!