It sounds like your 3 year old son is not actually ready to potty train even though he is peeing or pooping on the potty and wants to keep his diapers.
I think you should allow your 3 year old son to wear diapers when he wants or just let him go back to wearing diapers and using them instead of the toilet.
Your 3 year old son would be so much happier in diapers just like this little boy of around the same age.
Many 3 year olds and even 4 and 5 year old boys wear diapers and are not potty trained and that is okay.