Dependant dropdown lists

Hi all

Potentially tough question but I'm trying to find a way to build 3x dependant dropdown lists for a small database using the format of the control structure (see attachment).

The constraints are:
- Must be error proof, ie once all selections populated, user can't go back and change a prior selection (thereby showing an invalid combination) without clearing the prior one/two first.
- Can't use VBA due to experience of long term admin
- I'm looking for a better formula alternative to using OFFSET / INDIRECT in the data validation as there will be circa 1000 rows of these inputs (along with some other formula) which has just killed the sheet previously

I was hoping there could be some kind of UNIQUE and INDEX MATCH combo i could use in data validation, but a) I don't think it's possible and b) my Excel skills are no where near that level

Appreciate any help to offer!