What would be the next step? Why I was given a booster shot instead of the 3 doses?

About 3 years ago, I undergone blood screening for Hepatitis B and found non-reactive and there was a numerical figure in the result that reads about 700, I'm not sure what does it mean but they said it's the antibodies count. And so I was given a booster shot for Hepatitis B vaccination instead of 3 doses shots. What does it mean? Why was it that i was given directly the booster where in fact i haven't done yet with the 3 doses shots of vaccination. What should I do to determine if I am protected or not against Hepatitis B virus.