As other have mentioned, B to B to B (or A) seems to be the most optimal combination with motors that are currently being produced. It's counterintuitive, but in simulations, B boosters clearly outperform C boosters in 3-stage rockets. There are a couple of reasons for this. First of all, weight is the big enemy here. The first stage booster has to be able to lift the dead weight of the second and third stages, and their respective motors. An Estes C6-0 has a higher peak thrust than their B6-0, but a 3-stager suffers more from the increased weight than it gains from the increased peak thrust. But an even bigger issue is revealed if you look at their respective thrust curves:
https://www.nar.org/SandT/pdf/Estes/B6.pdf
https://www.nar.org/SandT/pdf/Estes/C6.pdf
A booster motor has two very simple jobs; first, to deliver all of its thrust quickly, and second, to ignite the next stage and then get the heck out of the way as soon as possible after it burns out.
As you can see, the B6 booster delivers a peak thrust of 12.14 Newtons, which is almost 2 Newtons less than the C6-0. But now look at the burn times. The B6 booster is over and done with in 0.86 seconds, while the C6 lingers on for another full second with a low-thrust tail-off. The type of thrust profile exhibited by the C6 booster is murder on a multi-stage rocket, especially one with 3 stages. In a 3-stager, the first stage booster is waging a life or death struggle with inertia and gravity, giving its all just to get the rocket moving and heading upward. Then the second stage takes over and does the real boosting. If the first stage motor is a C6-0, it delivers a nice kick at the start, but then prances around the ring, soaking up the applause of the crowd before burning out and lighting the second stage. If you are first stage booster motor, in your death match with gravity you cannot afford to give your opponent an inch. But that's exactly what the C6-0 motor does. The less muscular B6 booster wins the match by delivering its kick right away and then immediately handing things over to its tag team partner, the second stage booster. Working together, they deliver a much more devastating one-two punch, which sends gravity right down onto the mat. The B6 is much more of a team player than its muscle-bound cousin the C6 is, and in this contest, superior teamwork always wins.
