That https://en.wikipedia.org/wiki/Napier_Deltic is pretty interesting.
You would initially think that the ignition events would be evenly spaced, but that's not the case. For every delta triplet, the ignitions come rapidly one after another, close together in the cycle.
In that second animation on the page, showing the firing order among 6 delta piston assemblies, if you keep your eyes fixated on any of the six columns, you can see the three firing events. Always C, B, A order.