Fillable PDF with feet and inches in fractions

I need to create a fillable PDF that accepts measurements in  inches and fractions of inches.  I need the user to input 3 different values and then have the form calculate what the lowest of those 3 values is.  When I select any type of number format, it won't let me use the fraction format.  Any way to make this work?   

Example:

Field 1: 35"

Field 2: 35 1/4"

Field 3: 35 1/8"

Form should calculate in last field: 35"