3 days left of my birth control but havent started yet

From what I've read, this could be normal. Apparently your body will take a few months to get your period straightened out.
Technically, you could be pregnant, but that's just because the only 100% effective method is abstinence.
Nonetheless, wait another month or two, and see if you have any other symptoms

I have been on the birth control for 3 months now

The average rate of "a few" is 3 - 4 from what I've heard. If you're worried, just go get a test done.