I got a FUE hair transplant 3 days ago, when will I be able to smoke marijuana again without negative effects to my hair?

How can I advise you on something that may be harmful to your healing. Any smoking after a hair transplant reduces the blood flow to the scalp. Your call, of course. 

I recommend that you stop smoking until you are done with your post op medications.  Once you are healed and you do not take your pain medications, you can return to your normal routine.