Rule 51b.1 says that for more than three but less than six games in a block, the first three games make up the series. You can't use the last three games of a four-game block.
Basically, you take three-game chunks starting with game 1 as a series. One series starts with game 1, the next series starts with game 4, and so on. If there are leftover games (i.e., not a multiple of 3), they don't count towards any kind of three-game series awards. You'd get high game awards or special achievement awards like Dutch 200 or All-spare game, but that's it.
SH