Properly used, so-called A-frame ladders are by far the safest for window cleaning. They were designed specifically for the job by Shaftesbury Ladders in consultation with working window cleaners way back in the late forties or early fifties.
The splay foot gives a much wider base, so increasing stability and the point was designed to fit ON the corner of the brickwork at the side of the window. Modern H&S thinking has decided this is dangerous and so decree it should only be put INTO the reveal (the angle between the brick and the window frame)
Basic geometry tells you: any three points always lie in the same plane, so all three points of a 'pointer' will always be in contact with the wall/ground. An open ender has to be set in a very precise position to achieve the contact of all four ends firmly on the wall/ground and this is rarely achievable unless the building is on perfectly level ground.
There is no such thing as a 'three part' pointer, or if someone has made one, I wouldn't trust it.
A 3.0m double will be adequate for most two story buildings, with a 4.0, 4.5 or 5.0 for higher work, though the H&S frown on this.
AS to overreaching: if the ladder is set into the corner of the window, then when you reach to the other corner you are actually reaching backwards, rather than to the side, and so lessen the chance of overbalancing. Large windows should be tackled by doing one side at a time, and moving the ladder from one side to the other as necessary. Setting a ladder in the middle encourages the attempt to overreach, and there's nothing to counteract the tendency for the ladder to spin around.
Nearly forgot:
The rule is you must not climb a ladder any higher than the point where the top would be in line with the middle of your chest. Any more and you become very unstable.