Hi Rainer,
You are right - it is required to have a "common" control between two forks. This is because all teams, when all runners have finished, not only need to have run all controls, but also all legs between controls.
If two forks don't have a "common" control between them, then there will be a lot of different legs between the last controls in the first fork and the first controls in the second fork.
There will be many more different legs than runners on the team, so it will be impossible to say that all teams have run the same course legs.
In order to have a separate branch for the third leg of the relay, you create a "leg fork". In the leg fork, you designate leg 1 and leg 2 to take branch A, and leg 3 to take branch B. This is configured in the dialog window when you create the fork. This dialog window has instructions.
Also, in the "Sample Condes Event" file that comes with the Condes installation, has a course called "relay with leg fork", which illustrates this concept.
Don't hesitate to contact me again.
Best regards,
Finn