IMHO 3 days is too short... keeping in mind that there is a minimum of information and skill you need to get in and out of the water safely, but it's also not necessary to do a 6-8 week course (although it certainly can't hurt).
Do consider, however, that even with a longer duration course with a very good instructor that your diving education does not end after the course - even if you don't take anymore courses. The more you dive, especially with experienced divers, the more you will develop skills, knowledge, and experience. Don't be afraid to ask questions and do NOT be pressured into diving beyond your training level... take it slow and develop your experience gradually. Eventually you may be able to find an excellent instructor who can mentor you and also help fill in the holes in your diving knowledge and skills.
Good luck.