Using formulas and data logic in field rules

Using formulas and data logic in field rules

It would be very helpful to be able to use math or logic formulas in the field rules
Like
if the number in dropdown2 is > dropdown1 then show dropdown1 in singleline1

What I am trying to do is to have 4 input single line fields the user inputs numbers into each of the first  3 fields and I would like the form to automatically figure out the highest number and input that into the 4th single line field