Yes, this is normal. At 3.5 mo, if baby has total 5-6 wet diapers/24 hrs, active, gaining weight & feeding well, then 5-6 hr gap at night is ok. Babies sleep longer stretches at night and pass less urine then, but pass frequently in day. 2 urines from 8pm to 6am can be normal if daytime count is good.
Check: No fever, no crying while passing urine, no foul smell, soft tummy.
Next Steps
Monitor total urine count 24 hrs, continue 3 hr feeding, weight check.
Health Tips
If <4 wet diapers/day, persistent crying, fever, or urine smells foul - consult pediatrician.