There have been 62 two-legged, aggregate-goal series played in the history of the MLS Playoffs, including the four that are still ongoing. MLS started playing these series in 2003, when it was determined that a best-of-3 series was probably a bit of overkill — and also not particularly well attended.
In all that time, on just three occassions has a team headed into the second leg of the series holding a three-goal lead. As fate would have it, all three have included the Seattle Sounders.
The current series is, of course, one of them. The Sounders are leading FC Dallas 3-0, courtesy of an eight-minute stretch in Leg 1 in which Nelson Valdez scored once and Nicolas Lodeiro scored twice.