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For the original Lock-On, you don't need an activation code.
For the installation of Flaming Cliffs 3, you need to install Lock-on first, because FC3 is looking for the installation as a prerequisite (this is a license problem between ED and Ubisoft). You don't need to run Lock-On and can de-install it once FC3 was installed successfully.
You also need to install DCS:World for FC3, because FC3 also requires it to be installed in order to run FC3. This is, because FC3 is a DCS Module.
You can run FC3 with any DCS:World version, but I recommend to use the latest.