Hello I think 3 weeks of veg in a solo cup is about perfect. Some people will go longer and especially with a photoperiod strain this isn't a problem. However the more built up the roots get in there, the less happy they'll be and more likely to stress out. So I'd say go ahead and transplant into the final container. Personally I like bigger pots...bigger pots, usually bigger plants and bigger yields. Happy growing. ✌️