Alas, this is not really possible in MySQL. You can define it, but you need to call the two functions again.
MySQL does not allow you to use subqueries in the from
clause. If so, then this would be easy using a subquery. It is the only database (to my knowledge) that has this restriction.
In general, SQL does not allow you to refer to a column alias in the same select
where it is defined. So that is also not possible.
One way that you can get this done is using two views:
CREATE VIEW _v_test AS
SELECT
func_calculate_total(...) AS mnt_total
func_calculate_one(...) AS mnt_one,
func_calculate_two(...) AS mnt_two
FROM ...
CREATE VIEW v_test as
SELECT vt.*, (mnt_total - mnt_one - mnt_two) AS mnt_res
FROM _v_test vt;