I was in a room with someone this last week for 8 hours, who found out he was positive for covid. i had a pcr test done three days later which was negative. does that mean my vaccines and booster have stopped me getting it from him?

Yes, that's possible. But the incubation period can be longer than three days. Have another test in a few days, and continue to protect others by self isolating until then. Good luck.

Even if you test positive by PCR your risk of getting sick is miniscule. Your risk of getting infected is markedly reduced by vaccine and booster but not entirely eliminated.

The most consistent and accurate testing outcomes appear to be at 4-7 days after contact or symptom onset. So, you were tested a little early. The vaccine should mitigate any infection and protect against severity.