Aggregates and joined tables...

Hi all,
I have a problem using aggregate function SUM() ...
The platform is PostgreSQL 6.5.2 on i386-unknown-freebsd3.2, compiled by gcc
2.7.2.1 .
The story is that I need to join two tables with an aggregate function.
Here is a sample model :

Table A consist of ( fa1 , fa2 )
Table B consist of ( fb1 , fb2 )
Table C consist of ( fc1 , fc2 ,fc3 )

I want to write a query which looks like this :

select fa1 - sum(fc3)
from A,B,C
where fa1=fb1 and fb2=fc2
group by fa1 ;

unfortunately I get this in result:
ERROR: Illegal use of aggregates or non-group column in target list

Any idea ?
How can I have my result with only one SQL statement ?

Thanks in advance
Omid Omoomi
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