It's been a long time since I last played BOA but here we go...
to get maximum ambush effect you should have one or two regiments both sneaky types
meaning you shouldn't use regulars but irregulars (Indian warbands and Coureur battalions) instead (just in case you weren't sure).
and with a leader who is a good ambusher
This and that the troops they command are irregulars.
Three regiments in one corps seems like it is a bit much, but maybe not?
Not necessarily. The English may come with several regiments so you need some muscle too.
Next: usage. She he move toward an enemy position? or park somewhere in a province that enemies are likely to pass through?
Yeah, set an ambush where you anticipate the enemy will come but make sure to check the terrain type (you'll need woods and wilderness to succeed in ambush).
Offensive mode is needed.
Indians can be used to siege the outlying British forts for some nice victory points and that will deny the enemy some important supply.
My observations so far are that, you can actually ambush when you move into a province set to offensive posture so sittint and waiting for them to cvome to you is not always necessary
I don't remember that but ambushing was IIRC tricky business, so be careful.