perhaps I found a solution:
I added a new rule to wa server validation sb:
Validation Type: Required...
Server variable: ((((isset($_POST["field2"]) && $_POST["field2"] != "00")) || ((isset($_POST["field3"]) && $_POST["field3"] != "00")) && ((isset($_POST["field1"]) && $_POST["field1"] == "1")))?"":$_POST["field1"])
It seems to work.
Is there a better way to do this?
TIA
tony