Thanks for the question.From your description, it is evident that he had vasectomy about 3 years ago. Since then you had unprotected sex and that has not made you pregnant till now. So, there is no more need to worry about the effectiveness of the surgery. Vasectomy has got a success rate of more than 99% and the procedure is safe and reliable. The semen he ejaculates is sperm free and chances of getting pregnant are nil. You need not worry about vasectomy or think about any other contraceptive measure, as he had the surgery very long ago. There is absolutely no chance to get the sperm tube knot untie by itself. I hope that my answer helps. Best wishes.