Europe 1940 Second Edition Rulebook Page 22
(If you are upgrading a minor industrial complex to a major one, you may still only mobilize up to 3 combat units from that industrial complex this Turn.)
A good rule of thumb is that you can only do/use something based on its status at the very beginning of your Turn. Such as landing planes in a newly captured territory, going though a canal or using a Naval Base. At the beginning of your Turn the industrial complex is a minor thus only 3 units can be built.