It's recommended that kids stop using pacifiers when they reach 12 to 18 months of age.
But it's not really bad for 3 year old kids to have and use pacifiers but extended pacifier use can interfere with the 3 year olds speech.
So the pacifier use in the 3 year old kid may make it difficult for the 3 year old to begin to speak properly.
I would recommend trying to wean the 3 year old off the pacifier because there's not much benefit for the 3 year old to have the pacifier anymore at least during the day.
Pacifiers are good for babies but when they get past 2 years old there's not much need for them and they can sometimes do more harm than good.
But if your 3 year old still insists on the pacifier then I would not push the issue of removing the pacifier until they are nearing 5 years old and still using the pacifier.
If it keeps the 3 year old calm or helps the 3 year old sleep better at night then there's no harm in letting the 3 year old use the pacifier.