How to resolve notice : Undefined offset: 3 in user_node_load()
I'd guess you might be right. I think I have a different version of the user module from you (different line numbers), but user_node_load() adds stuff back in from the users table.

If you are missing a user with uid 3, you will get this error.

You probably need to update all of the nodes authored by user 3 to another user, the following SQL should do the trick (replace {new_user_uid} by the uid of the user you want to move the nodes to):

UPDATE node SET uid = {new_user_uid} WHERE uid = 3;