do the doctor believe you're having a miscarriage? have they continued watching your hcg levels? do you mean you did an ultrasound at 3/4 weeks? that's way too early to even detect a heart beat, of course it's possible, but it's almost hard to see it, even at 5-6 weeks. you should demand an ultrasound done again. and if according to you, your baby passed 3 weeks ago, then you're in need of a D&C so you don't get an infection from the fetus.
I would demand a scan for ur peace of mind. Some doctors push for surgery too soon. With my second son we didn't see a heartbeat till almost 8 weeks. Its your body don't let anyone push u into anything u don't feel right about. Good luck