aerodynamics - NASCAR drafting at Daytona

This year, the racetrack at Daytona has been repaved. The track was always faster than other tracks NASCAR raced at and several cars in a "train" were faster than single cars or smaller trains. This year is different, however, because two car groups are by far the faster than any other groupings. When a third car joins a tandem, all three slow down noticeably. Why is this? Why wouldn't it still be faster to have more cars pushing?