MySql sum elements of a column

 select sum(A),sum(B),sum(C) from mytable where id in (1,2,3);

select
  sum(a) as atotal,
  sum(b) as btotal,
  sum(c) as ctotal
from
  yourtable t
where
  t.id in (1, 2, 3)

Try this:

select sum(a), sum(b), sum(c)
from your_table