I am just lurking, but I unfortunately also had a 3rd degree tear (had a vacuum assisted birth) and I want to say it was maybe 3 weeks before I could sit without wincing and pee without stinging. I second the PP that suggested Dermoplast.
One thing that really helped me was one of those hemorrhoid pillows-- it was a round foam shaped pillow with a hole in the middle. That really took the pressure off while I was healing-- so just wanted to throw that out there.
Also, you will experience heavier blood flow when you increase your activity-- so take it easy for a while!