If you mean that you want the data in fields A and B only to be visible if the value in C is not zero:
If this if FileMaker 13 or newer, Put: YOurTable::C = 0 as the Hide Object When property for fields A and B.
In older versions:
If these are simple 1 row fields of data, set up a conditional formatting expression of:
YourTable::C = 0
and specify via "more formatting..." that the font size be set very large---such 120+ points if the text in the field is normally sized at about 10-12 points.
Use the same conditional formatting expression, but change the text color to be the same color as the background.
You can also right click on the fields and select conditional formatting select add and then change condition to formula is and then create a formula. probably c (does not equal) 0.