3) Online Auction System ER Diagram:

                                    +-------------+
                                    |    User     |
                                    +-------------+
                                    | user_id (PK)|
                                    | username    |
                                    | email       |
                                    | address     |
                                    +-------------+
                                            |
                                            |
                                  +---------+---------+
                                  |                   |
                          +-------+-------+   +-------+-------+
                          |    Places     |   |     Item      |
                          |   Bid         |   +---------------+
                          +---------------+   | item_id (PK)  |
                          | bid_id (PK)   |   | name          |
                          | user_id (FK)  |   | description   |
                          | item_id (FK)  |   | starting_price|
                          | amount        |   | current_price |
                          +---------------+   +---------------+