Just from a quick read and from what I've previously heard NO, if they do that then for 3 man flights:--
1/ more rest in Europe will be required. Ref 7.1.21.3D
2/all 3 Pilots need a P1 rating. Ref 7.1.7.2
And the current RPs "Manning Levels" stops them.
However
7.1.7.2 allows them to schedule Europe flights as 4 man but with 2 SOs because the Scheduled Sector Time is less than 15 hours. Lowest cost option for the flight in terms of combined salary, HOTAC and accumulated FDP.
7.1.7.2. Augmented Crew
The boarding of extra Flight Crew members for the purpose of providing relief at the controls. When four pilots are boarded for flights with a Scheduled Sector Time of 15 hours or more, a maximum of one pilot may hold a P2X rating. When three pilots are boarded for flights with a Scheduled Sector Time of 12 hours or more each pilot must be P1 rated on type.
While the 2 SO option is currently possible, I hope the new RPs tightens up the Manning Levels section to ensure that a crew has a maximum of 1 SO.