At 3 months of age territory marking would not yet be a problem, which leaves us with medical and behavioral explanations. Infection in his bladder or the outflow tract "urethra" can lead to frequent urination. He is also at an age when not understanding the potty training process can also be a problem. The best way to separate these two possibilities would be to have his urine checked by your veterinarian. If the urinalysis is normal then we would need to review your training routine. Please keep me informed.