3 weeks late is by no means too early. At 3 weeks late you would be anywhere from 6-8 weeks pregnant, and most women find out they are at 4-5 weeks of pregnancy. Make sure when you're testing you use first morning urine and a good test, such as First Response.
Negative 3 weeks after lmp? I would see a doctor. I know a lot of ppl get positive results at different times but surely it should have come up by now?