If the 3 year old wets the bed at night then yes the 3 year old should wear a diaper at night.
If the 3 year old stays dry through the night then there's no need for the 3 year old to wear a diaper at night.
Even when a 3 year old is potty trained in the daytime it's normal for the 3 year old to wet the bed at night and need a diaper.
So if your 3 year old is not dry at night then they should wear a diaper.
They could wear pull ups but from my experience pull ups leak especially on heavy wetters at night.
But diapers hold the pee in much better.
It's normal for kids to continue to wet the bed even until they are 17 to 18 years old.
I personally wet the bed myself up until age 17 and then I finally quit.
I wish I had diapers for the bed wetting at the time as waking up in wet sheets and clothes every morning did not feel good at all.