Limit Field input to "1", "2" or "3"

Thanks in advance for any help,

I have a form that is performing calculations and I need the input fields to be limited to the numbers "1", "2" and "3". I currently have Number formatted to "0" decimal places and the validate range set from 1 to 3. This will Change an entry such as "1.2" to "1" but when this number is calculated as an average, the "1.2" is acutally being calculated.

I need a way to keep anyone from inputing ANYTHING other than 1,2, or 3.

Thanks again!