There could be several ways of building such case, and many would heavily depend on how A and B work and the integrations and capabilities you may have enabled in your instance.
So the most agnostic solution would be to create a scenario D that has three steps.
Step 1: Run scenario A
Step 2: Run scenario B
Step 3: Run scenario C
Here, of course, I'm making the assumption that both A and B can be executed almost at the same time. Depending on how long it takes to run A, ie if it takes too long to wait to run B, then you could replace steps 1 and 2 with just one Python step where you trigger both scenarios simultaneously using the API.
I hope this helps!