IF Field 1 is blank, used field 2, if field 2 is blank, use field 3, **If field 1, field 2, field 3 are all blank, enter blank value/null

Try this

var text FieldOneTwo =
IF(Trim([Field1]) <> "", [Field 1],[Field 2]);

IF(Trim($FieldOneTwo) <> "", $FieldOneTwo, [Field 3])

The Trim is done just in case the data in the field was typed as spaces.