Plan B needs to be taken within 72h or it is not effective.
A HPT will not detect pregnancy until about 14 days after possible conception (unprotected sex).
I would take the plan b but you might need to call a professional just incase and verify.
If it says it can be taken up to 72 hours after sex, then you could still take it up until 230 am Tuesday. You need to wait until your period is due to test, but keep in mind that if you take the plan b you could cause yourself to have an early or late period, because it messes with your hormones. Also, if you were ovulating and your egg was fertilized within the last 48 hours plan b is not an abortion pill. Plan b is not 100% effective. You should see about getting on birth control and use condoms!
Plan B is most effective used immediately. I don't believe it will really do much of anything now, since it's been 3 days, though they say you can take it up to 72 hours after. The choice would be yours really. It's also far too early to even think about testing. You'll have to wait at least two weeks from the last time you have sex and test with first morning urine.