If you have multiple combinations, Then Create a Helper entity and Create A, B, C Attributes and then create a 2 separate association between main entity and Helper entity.Then By choosing Association A, B call a Onchange microflow and check for the combination in Helper Entity and set the Helper Object data in C attribute.
Hi Sangitkumar,
As per my asumption, If you enter a Value in A and B then the C attribute should reflect based on the A and B. For that Create a OnChange Microflow and Call it in the field of Attribute A and B and perform the Action.