Hi:
This can be done with Data Validation.
You'll need to set Data Validation on each of your three fields. Each field will require 4 conditions. For example, for Field1:
If Field1 is blank and Field2 is not blank or Field1 is blank and Field3 is not blank
For Field2:
If Field2 is blank and Field1 is not blank or Field2 is blank and Field3 is not blank
And for Field3
If Field3 is blank and Field2 is not blank or Field3 is blank and Field1 is not blank
A little repetitive, but it should do the trick....