Hi Palani,
System always updates the same confirmation category upon receiving the Order Acknowledgement through EDI and that confirmation category is derived from the internal confirmation category (1-Order Acknowledgement) in the settings IMG->MM->Purchasing->Confirmations->Define Internal Conf Categories.
However in your case, whenever you receive the order acknowledgement in 3 different stages, the data which you are passing in the IDoc will be updated except the Order Acknowledgement which will be replaced with AB in your case.
Please check the below info from SAP help
http://help.sap.com/saphelp_470/helpdata/en/1c/deffb0844f11d2b471006094b9476a/content.htm
Updating of PO When Several Partial Acknowledgments are Received
An order acknowledgment always relates to a PO item and not to individual schedule lines.
If a vendor initially sends a partial acknowledgment relating to only two schedule lines of an item with three schedule lines, you will expect further acknowledgments.
As soon as a further partial acknowledgment is received via EDI, "all old acknowledgments recorded manually or via EDI" are deleted. All old data is replaced by the data of the current acknowledgment.
Regards,
Asik.