Unique Combination

I have thought about this for days now (and nights) and I have simplified my problem below. (I even bought a book)
I have three tables: 1, 2 and 3.
Table 1 = a, b and c
Table 2 = x, y and z
Table 3 combines table 1 and table 2; ax, ay, az, bx, by, bz, etc.
Table 1 and Table 2 are both One to Many with Table 3.
The Table 1 field and Table 2 field are a ‘unique’ index combination.
My Problem:
It would be very handy to produce a list of the ‘unique’ index combinations that are not in Table 3. That is, the ‘unique’ index combinations that aren’t used yet.
You could do this by trial and error, but is there a Query you can run that will give you a result that you could then Append?
An Unmatched Query on the Table 3 / Table 1 contents will point out a complete lack of either a, b or c as will the same Query on Table 2 for missing x, y or z.
A query with the two non-linked tables will give every combination, but is this the best way to do it? It works by default when you use it to append table 3, but is it the best way to do it?