i would leave grapes and watermelons alone, i think you're just adding work there.
as for the cherries, i would sit him down, explain the pit situation to him and then have him eat a few in front of me, sitting down, one at a time. if he happens to swallow one, i'm sure he'd be fine, i've swallowed lots in my day. but i think if he's into spitting them out, i'd give it a try.
i'd be curious to see what other parents (ones that actually have 3 year olds) recommend though. maybe i'm wrong?