Can I be a little funny ? :) he is 3 years old - so he doesn’t wear a diaper - his parents make him wear one :)
First step - stop the night diaper especially if he frequently Urinates and you don’t frequently get up to clean. Diapers are for convenience to both the baby and you. Not only for you. So if it’s an inconvenience to him then stop using them at night. Thoda toh night ka sleep sacrifice karna padega aapko
Next Steps
If this doesn’t help then he may need a steroid cream to calm his skin ( steroids are good!!!! Esp when prescribed and used as PER PRESCRIPTION:) )
Health Tips
Always visit a dermatologist personally with the baby when it comes to baby issues. It is safer and treatments are more effective.
Hope this helps!