Version 1
round((toNumber(prop("One")) + toNumber(prop("Two")) + toNumber(prop("Three"))) / 3 * 100) / 100
Version 2
if(prop("One") or prop("Two") or prop("Three"), round((toNumber(prop("One")) + toNumber(prop("Two")) + toNumber(prop("Three"))) / 3 * 1000) / 1000, toNumber(""))