Your 3 year old son loving to wear wet diapers in normal.
A wet diaper does not actually feel bad to most babies or toddlers and the swelling up of the disposable diapers make it more comfortable to sit in.
Although it may seem uncomfortable to an adult that the toddler or 3 year old is wearing a wet diaper the disposable diapers when wet are not actually uncomfortable.
The SAP in the diapers hold in the pee so the skin does not really feel wet anyway.
But the swollen diaper is very comfortable.
The kid has been used to wearing diapers and peeing and pooping in them that it makes sense to the 3 year old that it could feel good.
If the 3 year old was in underwear then he may feel more comfortable but for now the diaper being wet is all he knows to be comfortable.
When I was 2 years old I was the same way about wet and dirty diapers.
I remember being 2 years old and refusing to get my diaper changed and I would run off and hide from my mom or dad when they had a new diaper in their hand.
This was in the year 1990 which was 30 years ago and the diapers back then were those plastic backed thick Pampers Diapers.
I loved wearing diapers as a 2 year old but hated having the diaper changed.
The reason your 2 year old loves wet or dirty diapers is because honestly the diaper feels good when wet or even poopy.
It might seem like it would be bad feeling to be in a wet or dirty diaper but actually it feels good to a toddler.
The squishy feeling, nice warm feeling etc feels great.
And the 2 year old does not know anything else because they are used to wearing the diaper and peeing and pooping in it.
Some toddlers hate wearing diapers whether clean or dirty and some actually like it.
I had to be forced to change my diaper as a toddler and I hated clean diapers but preferred when they were wet or dirty.