After 3 days it may well slow down enough that you think it has stopped, I have two brews that bubbled quite strongly for over 7 days, whereas another stopped after 3 days, yeast type and wort can make a big difference. However the chances are it is still working away quietly and I always give mine 14 days to let the yeast finish fermenting and clear up after itself. The only way to really know if it has finished is to check the gravity and if it is the same three days later then it has finished, but that uses a lot of beer up proportionally ( I don't risk pouring it back in) which is why I usually wait until day 10 before checking gravity.