Can an amended return be filed to add gambling losses as gambling winnings were already added? Is it worth doing for a $3,949 deduction?

Yes you can amend your return to include your gambling losses to the extent of your gambling winnings. You can only offset the winnings. If you just filed your original return I would let it get processed and then amend your return. If you filed your return a while ago then you should be fine filing your amended return now.