left join and is null sub query is too slow, plz help

i am trying got data from table a and table b
in a sub query. I have to do a left outer join and in where clause I have to check if the column1 is null.
but the query is taking 14 seconds, which should take less than a sec.

Any advice?

select * from tablea a
inner join tablec c on a.col1 = c.col1
left outer join tableb b on a.col1 = b.col1 and a.col2 = b.col2
where c.col2 = something and b.col3 is null;

Thanks for the reply.
I tired it, but I don't want to do an inner join as I need records.
We have added an index too, and instead of left outer join, used Exception join.
Which bring down the query run time from 13 to 6 sec, but still needs to be improved.