flowering and fan leaves

would trimming your larger and some medium size fan leaves during flowering focus more plant energy towards bud growth? or is it best to just leave it alone and let it do its thing?

It would take away from bud growth and overall feeding of plant. Leave leafs alone my friend.

leave em alone expecialy during flowering.

I'm 3 weeks into flower and fan leaves are starting to fall off White Widow aprox 43" tall, closet grow. Is this gonna be a problem?

magnesium deficiency

Assuming you have a good grow, the plant will prune her own leaves and they will turn yellow and fall off on their own. Never remove a leaf unless it comes off willingly and without force. If its ready to come off, it will literally fall off with the slightest tug.