does this work?
SELECT COUNT(x.*) AS countx, COUNT(y.*) AS county, COUNT(z.*) AS countz, (countx+county+countz) AS total FROM mytable1 AS x, mytable2 AS y, mytable3 AS x
Edit: No. but the following works
SELECT countx+county+countz AS total FROM
(SELECT COUNT( * ) AS countx FROM table1) AS x,
(SELECT COUNT( * ) AS county FROM table2) AS y,
(SELECT COUNT( * ) AS countz FROM table3) AS z