COUNT with set DIFFERENCE and UNION

Question

Fairly simple question that I can't seem to find an answer to - I'm working with unions and differences, and I would like to perform a COUNT on the results. Currently I'm having to pipe out to a file and "wc -l" the file (minus 4 for the Postgres printing). There's got to be a way to include COUNT...

SELECT tbl1.id EXCEPT (SELECT tbl2.id UNION tbl3.id); 

I would like to know the number of results after the set difference. Thanks for any input!