Hep B titers should be drawn 30 days after the 3rd dose is administered, ideally no more than 6 weeks. The titer level peaks during this time, after which the number could decline. The ref range is now anything >12.0 IU/mL. Titers CAN be drawn at any time following the third dose, but the results are not optimal. You wouldnt want to wait too long and be, say, 11.0f at 8 weeks when you may have turned up 12.0f at 6 weeks. You may still have adequate Ab to Hep B if exposed, yes, but most schools and health care systems follow CDC guidelines for confirmed immunity. Also, many schools and hc systems will accept just the three doses. (We do). Depends on the policy.
(I know this post is old, but it's sort of right up my alley and I hope my answer can help others).
Source: I am Employee Health Nurse at large hospital