I want to stop using diapers for my three year old in the night, but afraid that it would affect his sleep. Should I wait?

You have to take a chance and the time is right now. It is not an easy task to wean kids off diapers, but trust me this too shall pass. Also, since it's summer now, it is the right time for you to let your child be without diapers. However, remember to take him to the bathroom just before he hits the bed:)