Validation by Calculation

The field validation boils down to a boolean value, so you need a validation expression that evaluates to
Short answer:
  if ( if field1 = "x" and field2 = "y"; not IsEmpty(field3); 1 )

Longer answer:
  If you really want to strictly enforce validation, you need to take into account the user possibly changing the fields in a different order, as the validation only runs when the field changes. So you might want to add validations to the other two fields as well, so that the user can't go back and change field1 to a "z" without getting an alert as well. Might be overkill, depending on your application.

For such a headache, I suggest to use a custom dialog box to enter all 3 values in the same window,, using global temp fields. Yr control script will check values, and then issue a msg telling what's wrong and offering to loop or escape, leaving data unchanged, or setting the corresponding field from the temp ones.

I dont want to use a dialog box because most of these fields are from selections.  I have a activation process that will check whats filled out.  I normally put the validations in there, but I thought there might be an easier way.

true, this dialog box is missing check boxes and popups...
could you use a plug-in which executes a script whenever a given field is modified? (softs4humans event plug-in)

Thats it exactly!