Yes it's possible that a 3 year old would use a diaper and pretend they have no control even if they actually can hold it.
By 3 years old the child should be able to hold their pee for awhile and at 3 they have gained some bladder control although it's still a bit weak.
A 3 year old or toddler may use a diaper instead of the toilet when they are wearing a diaper as they may not be ready to potty train or they just prefer to use the diaper and like the feeling of the diaper.
The 3 year old has been wearing diapers all their life so they are used to it and going in the toilet is a new concept that they have to get used too.
Toddlers use diapers even when they can hold it because toddlers have been so used to wearing and using diapers since birth.
Since the the toddler was born they were put in a diaper and were supposed to use the diaper to pee and poop in and so suddenly the toddler is expected to not use the diaper and use the toilet instead.
The toddler uses the diaper as it's a habit they were so used too so you have to break that habit by reminding the toddler to use the toilet instead of the diaper.
Still if the toddler is not fully ready and still uses the diaper instead of the toilet then you should keep them in diapers for awhile longer and try again with toilet training later when the toddler is more ready.
The point of making your kids wear a diaper is to keep the kids clothes clean and also to keep your furniture, carpets etc clean.
If a kid who is not potty trained does not wear a diaper then they will pee and poop in their pants and get it all over them, the bed, your couch, the floor, in your car etc.
Without a diaper on the kid who's not potty trained there would be a huge mess and it would be unhygienic and unhealthy for the kid to be peeing and pooping on themselves and in their clothes.
When babies are born they have no bladder or bowel control and even as the child gets older and reaches 2 to 3 years old the child's bladder is still weak so they can't hold it very long.
So a child who is wetting their pants or pooping their pants or those who are not potty trained will need to wear a diaper.
Even if the kid does not want to wear the diaper they must be made to wear the diaper if they are not potty trained.
Or if a kid is incontinent or bed wetting then they should wear a diaper to keep them healthy and clean.
Some kids potty train by age 18 months and some are not ready until 2 to 3 years old.
Then again some kids may not be potty trained until 4 and even 5 years old.
Every kid is different and will eventually potty train and want to stop wearing diapers but until then the child must and should wear a diaper.