I know Skirmish is a SA
Skirmish in one space is one SA. Skirmish in three spaces would be three SAs.
Howe's ability says:
Before executing a British
SA 1rst lower FNI 1 level.
It does not have any requirements regarding why you are executing that SA. All it says is that whenever you execute a British SA, completely regardless of whether you do it together with a Command or because an Event tells you to, you first get to reduce FNI by one level.
The "Von Steuben" Event allows the British to Skirmish in three spaces. The way Skirmish is defined by rule 4.2.2, that means three separate SAs - or rather up to three, since the British can only Skirmish in spaces that have both British Regulars and Rebellion
This Event can be extremely powerful for the British... if the Rebellion Factions give them the opportunity to play it, that is.