It was all rather fun, and thanks to all of you for your great and witty answers to yesterday’s quiz—we’ll have to play again sometime.  Congratulations, commenter ChipSF, for perseverance and stellar detective work in tracking down the correct answers.  A special nod to Anonymous, who said the thread was ‘isms’—narcissism, imperialism, and socialism.  
But indeed, Mogens Tvede was the thread that tied the three pictures together.  Here’s as much of the story I could piece together without a visit to the library:
 The gardens of the Chateau de Brantes in Avignon, a late design by Mogens Tvede, for his cousin.
Long before Lee Radziwill, there was another famous Princess Radziwill, Dolly )1886-1966).  She was a patron and friend of artists, writers, and designers from Cocteau to Berard to Dior, and a major figure in International Society in the first half of the 20th century.  The portrait below, by Alex-Ceslas Rzewuski gives a hint of her stylish charms.

Prince Radziwill died in 1920, and the Princess soon remarried, to the Danish architect and painter, Mogens Tvede (1897-1977).  They lived by all acounts in splendor in a large town house near Les Invalides in Paris., where their circle of friends included Nancy Mitford, whose portrait was painted by Tvede. 
Although I am unable to find a great deal of information, it appears that the Tvedes were friends of the Standard Oil magnate Walter Brewster Jennings.  1927 found them as the house guests of the Jennings at their Villa Ospo on once fashionable Jekyll Island off the Georgia coast.  As a result of that visit, Tvede was commissioned the next year to design a house there for Frank Gould, the Villa Marianna (below).
By 1930, Tvede was associated with society architect Mott Schmidt, a designer of cool, restrained and very traditional Georgian houses.  Although Schmidt loyalists protest loudly at the idea, one presumes this was a marriage of convenience, it being unlikely that Tvede had a license to practice in New York.  That year they designed a house at Cold Spring Harbor on Long Island for Oliver Jennings, son of Walter Jennings.  It is likely, given Tvede’s friendship with the Jennings family, that Tvede was the lead designer on the project.  It is certainly unlike anything else Schmidt designed, a house in the modern Scandinavian neo-classical style, bold and radical, the very antithesis of Schmidt’s usual work.
After traveling down a nearly mile long drive, one drove through a central arch in the garage building facing the main house across the gravel court
The entrance front of the house reminds one of the work of Emilio Terry
A two story entrance rotunda led to the drawing room
The double height drawing room was a masterpiece of modern neo-classicism.
The water front of the Dark Hollow

A holy grail for this post was to find a picture of the elegant octagonal pavilion at water’s edge on this estate, but it eluded me—I thought I had saved one from a real estate ad last year, but no can find.
And, just for discussion’s sake, here is a photo of Dunwalke Farm, the Douglas Dillon house in New Jersey, designed by Mott Schmidt in 1936, and extremely typical of his work.  I rest my case.
Photographs of Dark Hollow, photographer uncredited, from a 1933 issue of House & Garden.  Photograph of Dunwalke Farm from Mott B. Schmidt, Architect.