Can I be pregnant after taking plan b one-step exactly at the 72 hour deadline?

You can get pregnant even if you take Plan B the same day as you had unprotected sex, as it is never guaranteed to be 100% effective. However, it is most effective the sooner you take it. If you take it at the end of the 3 day window, then there is more of a chance that you'll become pregnant than had you taken it the same day, but it may still prevent a pregnancy.

If you got your proper period the day before taking Plan B, & it was scheduled & on time, then it is less likely that you will become pregnant.

You should be able to do a pregnancy test now, given the date of this post.