It clearly states in official guidance to LPAs that enforcement "might not be appropriate for a minor or technical breach where no harm has occurred".
A couple of square feet over the 3sq m is not going to raise any issues.
As mentioned above, don't apply for a LDC because the council will be duty-bound to refuse it if the drawing shows it over 3 sq m. You would also have drawn their attention to the slight excess, and if any neighbour did complain once it was built, the LPA would take a dim view of your transgression and might be more inclined to press the issue.
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