While some extensions to the notation have been proposed in the past, this is a common group of algorithms and the notation is already somewhat being used.
I propose that [A,B,C] = [A,B] [B,C].
After expanding, a cancellation in the middle gives A B A' C B' C'.
For example, [l', RUR'U', r] is a common 4x4x4 last layer algorithm.
This notation can be extended further -
[A,B,C,D] = [A,B] [B,C] [C,D] = A B A' C B' D C' D'
but I am yet to find a use for it :)
Finally, just as [A,B]' = [B,A], [A,B,C]' = [C,B,A].