You could trim some of the lower branches if they look like they are just using up energy and will never really flower properly beyond the occasional "popcorn" bud. This is usually the case as these branches receive less light than the rest of the plant and thereby become energy sinks. Since you are almost 3 weeks into flowering, you can probably tell which branches would qualify for removal.
On the other hand you could leave them on and then harvest the plant in two goes, the top half first and then the lower half.
It's up to you but personally I don't think that it's worth the wait. In case you have a mixed indica/sativa grow it's another deal, then you could easily leave the lower half of the plant to flower on while the sativas mature. If all your plants will finish at the same time, then you could consider removing some of the lower branches so that the plant will direct more energy into the larger main colas.
On the other hand you could leave them on and then harvest the plant in two goes, the top half first and then the lower half.
It's up to you but personally I don't think that it's worth the wait. In case you have a mixed indica/sativa grow it's another deal, then you could easily leave the lower half of the plant to flower on while the sativas mature. If all your plants will finish at the same time, then you could consider removing some of the lower branches so that the plant will direct more energy into the larger main colas.