Plan B effectiveness 3 days before ovulation?

What makes you say you were 3 days before ovulation?
If it was an app or website, they are just guides. The only way to be certain is to record your fertility symptoms each day for a couple of months (worth doing to learn how your body in particular, works).
If you didn't do that, & your periods are regular, then you were likely 1 day before ovulation.
Most women ovulate somewhere around day 14 of their cycle (day 1 is the first day of your period)
If you're a few days either side of that, then Plan B is not the recommended form of emergency contraception.
This is because Plan B works by delaying your ovulation until the sperm leaves your body. If you're close to or already ovulating, then it can't do that, so you'd need one of the other methods of emergency contraception; either the other emergency pill called Ulipristal Acetate (Ella), or to have an IUD fitted by a doctor. Either of those things can be effective if done within 5 days/120 hours of the unprotected sex.

You don't necessarily get symptoms from Plan B. It varies between people, & when in your cycle you use it.

Plan B can cause thicker mucus, but so can pregnancy.

If it were me, I'd use Ella. Only you can decide.