hi Paul,
this depends on the relationship between the tables and the data in those tables. if a star schema (dimension table plus a bunch of fact / details table) then you'd link from A to B and A to C using the part number. if it's linear then A to B to C.
first example could be (star schema)...a star schema you can think of as one driving master table and a bunch of details tables attached to the master table.
A= parts catalogue master table
B= customer orders
C= suppliers
second example could be (linear)
A= customer orders
B= product table
C= product type / desription table (i.e. a table which further describes table B)
i would check with your dba to see if they've got a schema diagram.
cheers,
jamie