How long do u bleed at 3 weeks or less?

About the same as a normal period. Maybe slightly heavier, but really, it's pretty much impossible to tell the difference between a late period and a very early miscarriage.

Chances are you won't even show a positive test. Because doctors estimate time of conception approx. two weeks after your menstrual cycle (although this widely varies between one woman to the next). And they count your weeks starting from your last period so you wouldn't even have missed a period and also would only been 1 week along. But, since a test usually won't show positive that early then most women don't even notice early miscarriages they just think they're having a heavy period or even normal period slightly heavier. Unless you mean you're 5 weeks pregnant 3 weeks since conception in which case it would still be about the same you might show a positive test but it would more than likely still resemble a period this early.