Thank you for your post.
Your manual is available through the Help menu (Contents and Index).
The validation formula for Field 1 would be:
IsEmpty (Field 2) and IsEmpty (Field 3)
I don't have FileMaker Pro 5.5 on my machine, but check the options (if available) for "Strict. Do not allow user to override validation" and "Display custom message if validation fails." You may want to put in the message "Information already exists in Field2 or Field3"
Do the same for the other two fields.