assignment

  1. I’ve been working my assignment all day.
  2. I’ve been working at my assignment all day.
  3. I’ve been working with my assignment all day.
    Please correct the above.
    Thanks.

2 is the correct form.

[color=blue]I don’t like any of the above. I would only say ‘…working on my assignment…’.
There is a difference between the two varieties of English.