Its not too late at all to defoliate. Some people will defoliate before the 12/12 flip to open up budsites and create less shaded spots, and then do an additional defoliation after the flip. If you chose to defoliate it probably will still help the budsites since youre only 3 weeks into flower, but it'll also help bring your humidity down...less canopy..less moisture being generated by the foliage. If you take a small % first 10-15% you can always take away more, so id start small..evaluate the difference then clip more if you feel needed.
Happy Flowering and love the grow box ✌️