need sql select help with multiple tables

Hi, I have a little problem that I'm sure most of you can solve with your
hands tied...here it is:

I have three Paradox tables, vdb01.db, tblExpenses.db and
tblOtherRevenue.db. vdb01 contains a list of vehicles and their
descriptions. The primary key is called ReferenceNumber.  tblExpenses is a
table describing all of the expenses for each of the vehicles in vdb01. This
table includes a foreign key corresponding to the ReferenceNumber field in
the vdb01 table. tblOtherRevenue is laid out similiar to tblExpenses except
it contains information about the revenue corresponding to each vehicle.

What I want is a query that will return a single row for each vehicle that
contains the Reference Number, description, sum of all expenses for that
vehicle and a sum of all revenues for that vehicle. Feeling quite clever I
wrote the following:
SELECT A.'ReferenceNumber',A.Description,Sum(B.'Amount') AS
TotalExpenses,SUM(C.'Amount') AS TotalExtraRevenue
FROM (vdb01 A LEFT OUTER JOIN tblExpenses B ON A.ReferenceNumber =
B.VehicleNumber)
LEFT JOIN tblOtherRevenue C ON A.ReferenceNumber = C.VehicleNumber
GROUP BY A.'ReferenceNumber',A.'Description'

However, because of the 'cross connecting' that the joins do I don't quite
get what I want. For example if a given vehicle has 3 expenses and 2
revenues, then the sum field for expenses ends up being 2X the real expense
sum and the revenue sum ends up being 3X the real revenue sum.

I would be extremely grateful to anyone who can help me out.

Thanks,

Brad