Updating list from diferent lists based on unique ID

Hi, 

I run out of ideas of how to fix my problem. I would like to have 3 lists. Lets name them

ListA 

ListB

ListMain - combaining records from ListA and B

The idea is:

- Anytime something new is created on List A or List B it is copied to ListMain - this works fine.

- Anytime something is updated - it should also be updated on ListMain - and this is the problem, since the [ID] column is not unique and does not match. (There is a row with ID = 1 on both List A and List B. )

I though I need to make new columnt on all lists named [UniqueID] that insted of [ID] = 1,2,3, etc. stores 1A,2A,3A for List A and 1B,2B,3B for List B. With this i would be able to update any record in MainList.

When I use power automate I can only use standard [ID] to update my MainList, and it doesn't work correctly. Any ideas how to approach this?

Best regards