Is first degree heart block is a problem for an aviation medical class 3 physical?

Generally Not. However, what medications are you on. The FAA is very concerned about the Meds. AOPA has assistance that can answer your Med approval questions. How is your EKG, sometimes the Aviation Medical Examiner will need to see the recent EKG. AOPA is good at their assistance before the AME examines you. Remember that if the AME sees something, they are required to report it; so do some prep work.