There is no need for a comma between "b" and "c" in the series "a, b, and c". The comma is used to separate items in a series, but it is not necessary before the final item. In this case, "and c" is the final item in the series, so no comma is needed.
People say that the phrase "the last before" is not necessary because it is redundant. The word "last" already implies that something comes before it. Therefore, the phrase "the last before" is considered to be unnecessary and can be omitted.