changing cell color based on changing values

This may have been answered on here but can not seem to find it. My situation is I have values in A1,A2 & A3 that are like counter reading so the value is always changing. What I am looking to do is change the cell color if one of the values is over 500 from the other two values. Say A1 is 3000, A2 is 3250 and A3 is 3500. I would like the cell for A3 to change color.