Storing multiple values in the same field suggest a less than optimum design for your table. Storing multiple values in two fields of same table? Even more so.
It's easy to determine that Ref1 appears in both fieilds. It is not so easy to determine that Ref3 does not appear in Field2.
A custom function could be devised in FileMaker Advanced that returns every value listed in Field3 that does not appear in Field2. That requires FileMaker Advanced to create and install in your file.
A looping script could loop through the values in Field3 and return every value in listed in Field3 that does not appear in field2. This does not require FileMaker Advanced.