All babies are different and sleep different amounts during the day and as long as the baby is getting enough sleep then it's usually just fine for the 3 month old baby to sleep all day.
If you're worried you can let the babies doctor know about the all day sleeping that the 3 month old is doing just to make sure nothing is wrong.
Sometimes sleeping all day could mean a problem or a condition that the baby might have but sometimes it's just a normal thing.
As the baby gets older they'll tend to sleep a bit less during the day and normally 3 month old babies will sleep 5 to 8 hours per day.
But some may sleep more if they're more tired.