Who can move first in this situation?

Your answer is correct.

In this situation, you have the right of way because the red car is entering the main road from a driveway or private road (so it must yield), and the blue tractor is behind you and also on the main road but further away. According to German traffic rules, vehicles already on the main road generally have priority, and vehicles joining from a side road or driveway must wait.

So, you can move first, then the blue tractor, and lastly the red car. Recognizing who is on the main road and who is entering it is key for these priority decisions.