Depending on the viscosity of the fluid filling the buckets after the cup labled 3 has filled I believe 5 may be the next bucket filled. Say the fluid is water, 5 would fill next depending on the rate of wich the water is flowing from the spicket due to water streaming along the surface area of the pipe and reaching cup 5 as it has the closest proximity to the first cup being filled (cup 3). As I said this only is possible assuming the fluid has a viscosity simmilar to that of water.
RE: Which cup will fill up First ?