Hi I am 3 weeks late on my period and I spotted like one light spot of blood about a week ago, I took a at home pregnancy test about 3 days ago then I went to the doctor the next day and they did a urine test and it was still negative.. is it possible for me to be pregnant?

Yes. Request a blood test.

And it's pregnant, not pregnant - just so you don't embarrass yourself.

It is still possible, but again it may or could be other things.
So you need to return to the doctor for additional tests

hi i am 3 weeks late on my period and i spotted like one light spot of blood about a week ago, i took a at home pregnancy test about 3 days ago then i went to the doctor the next day and they did a urine test and it was still negative.. is it possible for me to be pregnant?

It's possible, but most likely you would have had a positive result by this time if you were. Since you are a teen, it could be simply due to irregular cycles that are not uncommon in the teen years. Are you using any sort of hormonal birth control? That too can sometimes cause irregular bleeding.

I would wait until next month and see what happens when you would expect your period. Test again at that time if necessary.