Thanks Mel,
No apologies required, it seems the whole history of this development is a real can of worms and certainly makes my brain hurt.
I think that your understanding of it makes more sense to me, in that the C2a first flew as a Parasol on July 24, 1931 but was found to be deficient in its climbing performance and this led to the fitting of the additional small lower wing. Then this Sesqui-wing modification was given the designation Do C3a and made its maiden flight on September 18th, 1931. This seems more sensible than the Airwar's description that the C3 flew First as a one and a half wing first, but had its wings clipped the following year to make it a C2, but who knows!!