sub-select with aggregate

Hi
I want to perform query looking like this:

select user_id, a/sum_a as percent_a, b/sum_b as percent_b
from users join (select   group_id,   sum(a) as sum_a,   sum(b) as sum_b  from users group by group_id) X using
(group_id)
where group_id=3;

This query works, but very slow. Subquery with aggregate is performed 
for all table rows instead of group_id=3.

I wish I could write it like this, but it doesn't work at all:
select user_id, a/sum_a as percent_a, b/sum_b as percent_b
from users U, (select   sum(a) as sum_a,   sum(b) as sum_b  from users where group_id=U.group_id) X
where group_id=3;

I don't want to make it as single query with "group by", because it has 
a lot of fields and "group by" would be very long.

Regards,
Tomasz Myrta