with numbers as (
SELECT (SELECT value FROM ....) AS NUM_1,
(SELECT value FROM ....) AS NUM_2,
(SELECT value FROM ....) AS NUM_3
FROM DUAL;
)
select num_1,
num_2,
num_3,
num_1 + num_2 + num_3 as total
from numbers;
You have to make sure that your (SELECT value FROM ....)
queries return exactly one row otherwise you'll get an error like "Single row sub-query returns multiple rows".