Solved it !!
I knew that all the KEY fields in a database table are shown up automatically in the 'Lock parameter' tab of the lock object. However, mine was a structure ZPROJ instead of a database table.
I was not aware that DDIC structures in SAP also have keys. The 3rd field which I added was not marked as a key parameter in the structure and that is the reason it was not showing the lock object.
I switched on the key attribute for the structure from Extras -> DB Attributes -> Key On/Off. Then marked the 3rd field as key and that's it !!!
Closing the thread 