Enter a number, output a character

1. What is the problem? Be very detailed.

I am using a numeric keypad to enter data in inclement weather. We are attempting to touch the tablet screen as little as possible. I would like to enter the sex of a species as either 1,2, or 3 in an integer type question and have the output after data download appear as a character either M,F, or U.

2. What app or server are you using and on what device and operating system? Include version numbers.

Using xlsforms to design the form and kobotoolbox to deploy the html form.

3. What you have you tried to fix the problem?

Have tried if then statements in the calculation field but can't seem to get the form to function with these.
For example: if(${Sex}=1,${Sex}=M) or if((${Sex}='1','M',if(${Sex}='2','F',if${Sex}='3','U')

4. What steps can we take to reproduce the problem?

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.

Why don't you use a select one question?

I had thought about that, but select one involves more interaction with the number pad and potentially the touchscreen.

I think it's just 1 touch on the screen, which is the least.