S.Z.
Dont over think it! They are 3 and will be thrilled with anything! I would say the more rigid you try to make it the less fun they will have. Let them just play in the water. Bring bouncing balls and the squishy water filled balls made for the pool. If you can set up a sprinkler that could be fun. If someone can keep up on the poped balloon trash, so they dont eat it, trying to sit a one of those big balloons filled with water might be fun for them!
Good luck