At 3 yrs, your child is old enough for a conversation on the subject. He is now a big boy, nappies are for babies, no peeing in bed, peeing in toilet like big people, etc etc.
Put plastic cover, towel, sheet, another plastic cover, towel, sheet on his bed. (So if there is an accident in the night, you whisk one layer off and the bed is ready.)
There will be several accidents at first but hopefully he will get the message. Tell him each time that it wasn't good that he peed in bed (not in an angry way, of course). Personally, I don't agree with "Ignore the accidents, clean up but don't say a word".
Don't start with lifting him to toilet in the night. If he wets the bed for a couple of nights in a row and you feel he won't go through the night without a wee for a while, then think about such measures.