Realistically, if there is going to be 3 sleeper services to London then something has to give from at least one of the other sleepers in terms of major schedule changes.
As highlighted, the Edinburgh and Glasgow Sleepers are popular as it allows a late departure from London, whereas the Highlander currently is more about connectivity.
So if the sleeper did split into 3, what would realistically work?
Going by the above, the only solution would be to join the Edinburgh and Aberdeen sleeper at Edinburgh, equating to 12 coaches (assuming the sleeper from Aberdeen runs abbreviated at 4 coaches as apparently planned). This way the Aberdeen portion can leave slightly earlier at night, and arrive later in the morning, meaning folk from Leuchars, Dundee and Fife can be onboard before midnight going to London, and off after 7am the next day - possibly increasing the liklihood of passengers.
The Fort William portion then meets the Glasgow Central portion at Edinburgh, allowing the Fort William schedule to retain the passenger links to Glasgow Queen Street and Edinburgh where the seated sleeper currently serves as a "day train". Then both sets run as 14 coaches, including the Fort William seated and lounge coaches, and into London.
Meanwhile, the Inverness portion can freely run on its own as 8 coaches all the way to and from London.
Overall, the Edinburgh and Glasgow sleepers would be able to maintain their late departures and early starts without too much faffing about with the times, whilst the Fort William sleeper maintains it's day service from Edinburgh to Fort William via Glasgow Queen Street. The only sleeper suffering from time alterations would be the Aberdeen portion, which is already in decline, so earlier departures and later starts could possibly open the door for more regular leisure travellers. However, even none of these "realistic" suggestions seem realisic (if that makes sense), so the most idea solution in the event of a move to Kings Cross would be for Network Rail to invest in the station infrastructure to allow 2 platforms to accommodate 16 coaches - the current sleeper times as a whole are fine as they are, which would only lead to complications if a 3rd sleeper was introduced as a breakaway from either the Highlander or Lowlander alternatively.