It;s okay to change a 3 year old boys diaper in public or even a 4 year old boys diaper in public.
If your 3 year old or 4 year old is still wearing diapers and they are in public in the park or anywhere else and need a diaper change then you should change their diaper even if it's in a park in public.
The 3 year old or 4 year old may be embarrassed or may not be but if so you could change them in the car or in the bathroom.
But if the diaper needs to be changed then you should change it wherever it's convenient.
I've changed my 3 year olds and 4 year old boys diapers in the park on the park bench, on the ground and on picnic tables.
My 8 year old son wears diapers as well when out in public just in case he can't make it to the toilet and I change his Gentle Steps size 7 diapers in our van.
He has trouble getting them on just right.