If i have only been taking 150mg of wellbutrin xl, once a day for 3 weeks - can i just stop taking it? or is there a weaning?

You can just stop it because you have used only 3 weeks. In case of doubt, you can take q 2days for 2 days then q 3days for 2 days then stop.

Weaning is necessary, however please discuss with the prescribing provider. Is Wellbutrin not working; three weeks isn't time enough? Have you been were you prescribed other medication? Please consider a treatment plan which includes what you consider to be the problem(s), goals you wish to achieve, and then consider medication and other treatment options along with outcome measures.