I'm a little confused, because 3 weeks is 21 days, and you said you were supposed to have the stitches removed 7 days after surgery? So, was the surgery 3 weeks ago, or one week ago? When did the stitches go in, 3 weeks ago or 1 week (7 days) ago?

If the stitches have been in for 21 days, then the only way to get them out is in a doctor's office, since I'd think they have begun healing into your body which is why you've having a hard time removing them.

I had stitches once and they were painful to remove; I certainly couldn't have done it to myself!

Well, my urologist said the stitches are biodegradeable! I guess that I have to make an appointment to get them remove!

If the stitches are biodegradable, they may dissolve into your body and just disappear, but you'll need to confirm that fact with your doctor. If that's the case, then they don't need to be removed. However, if they are not truly biodegradable, then then need to be removed asap or they could cause infection.