Conditional Formatting based on Non Blank Cells

[Ashley1432]

I have a form for users to fill out and I only want them to fill out one of
three categories. So if they type something in one category I want the other
two categories to grey out. How can I do that?

 

[T. Valko]

Need more specific info.

if they type something in one category

And what cells would they be typing into?

I want the other two categories to grey out.

And where are these categories?