Of course, there are used constrains.
Gear A is fixed - it does not rotate or move.
The Disk has 3 holes. Central hole mates to Z axis of the fixed Gear A, so the Disk can rotate only. The other 2 holes are used for mating to Gear B and Gear C.
Z axix of the Gear B mates to the hole B, and Z axis of the Gear C mates to the hole C. So, the gears B and C can rotate arount Z axis, and also make "planetary movement" around the fixed Gear A (by rotating the Disk).
Gear B has rotation constrain to the fixed Gear A in ratio 0.5.
Gear C has rotation constrain to the Gear B with ratio 1.
The problem:
If I pick the Disk and rotate it, gears B and C does not rotate to each other.
If I pick the Gear B, it rotates correctly around the fixed Gear A, but the Gear C does not rotate.
If I pick the Gear C, it rotates incorrectly with respect to the the Gear B, and also the Gear B rotates incorectly with the Gear A.
I have to rotate the Disk and transfer rotation of the Gear B (which is relative to the fixed Gear A) to the Gear C.
I think it will be more clear if you will download and open the assembly to see how bad it works.
Just one more comment - the Gear C is not in conflict with the fixed Gear A, it is above it.
