From the sounds of it you need to think it through...
If Im understanding you right, you only really need a coplue of tables:
1 - Parts table
ref (autonumber)
PartCode
PartType (x,y or z)
PartDescription
Then you have an 'assemblies' table with
ref (autonumber)
Part1
Part2
Part3
You build aform based on assembines. Each on the parts are a dropdown based on the Parts Table, but with different source criteria
Part1: select PartCode, PartDescription from PartType where PartType='x'
Part2: select PartCode, PartDescription from PartType where PartType='y'
Part3: select Partcode, PartDescription from PartType where PartType='z'
Now, when you create a new record, you select the three part types, and that's it. Whenver you need to know the reference for the assembly, you can generate it as you go:
=[Part1] & "-" & [Part2] & "-" & [Part3]
You can use this on forms, reports, queries, etc.
Note: if you think you may have more (or less) than three parts things can get more complex. You are looking at a Many-to-Many relationship, which is perfectly possible, but a bit more complex.
SeeThru
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