It should be stutter, not flicker, that bugs you at 24fps....most theaters project with three or four blade shutters, so the flicker itself is above the flicker fusion threshold (72/96hz).
This comparison is useless since we don't know the lenses or other settings. The Scarlet should have done better. The Red MX and Epic look fine at 1200+ ISO. That said, the 5DIII is a true low light monster. If you turn highlight tone priority off (which results in clippy highlights), you can shoot 10000 ISO and it looks fine. With f1.4 lenses, it's pretty incredible. The availability of affordable 24mm f1.4 primes (≤17mm f1.4 primes for APS-C are harder to find) make the 5DIII an incredible camera for low light narrative.