My son complains about his penis being in pain every time he has an erection.he is 3 years old.my husband says that its normal.

Likely true: I doubt if a 3 yo can either understand an erection or be articulate enough to describe the different sensations it will cause. The process can occur from birth until old age. If you comment at all it would be to encourage him to keep the issue private. (like toilet activities) If you respond to it in fear or embarrassment it confuses the kid.