Passage: On 18 August, Admiral Scheer attempted to repeat the 31 May operation. The two serviceable German battlecruisers (Moltke and Von der Tann), supported by three dreadnoughts, would bombard Sunderland in an attempt to draw out and destroy Beatty's battlecruisers. The rest of the fleet, including Thüringen, would trail behind and provide cover. British signals intelligence informed Jellicoe of the German departure later in the day, and he sent the Grand Fleet out to intercept the Germans. On the approach to the English coast, Scheer turned north after receiving a false report from a zeppelin about a British unit in the area. As a result, the bombardment was not carried out, and by 14:35 on 19 August, Scheer had been warned of the Grand Fleet's approach and so turned his forces around and retreated to German ports.On 25–26 September, Thüringen and the rest of I Squadron covered an advance conducted by the second commander of the torpedo-boat flotillas (II Führer der Torpedoboote) to the Terschelling Bank. Scheer conducted another fleet operation on 18–20 October in the direction of the Dogger Bank, though rudder damage prevented Thüringen from participating. For the majority of 1917, Thüringen was assigned to guard duty in the German Bight. During Operation Albion, the amphibious assault on the Russian-held islands in the Gulf of Riga, Thüringen and her three sisters were moved to the Danish straits to block any possible British attempt to intervene. On 28 October the four ships arrived in Putzig Wiek, and from there steamed to Arensburg on the 29th. On 2 November the operation was completed and Thüringen and her sisters began the voyage back to the North Sea. A final abortive fleet sortie took place on 23–24 April 1918. Thüringen, Ostfriesland, and Nassau were formed into a special unit for Operation Schlußstein, a planned occupation of St. Petersburg. The three ships reached the Baltic on 10 August, but the operation was postponed and eventually canceled. The special unit was dissolved on 21 August and the...

Question: What units was the cancelled Operation Schlubstein comprised of?
Thüringen, Ostfriesland, and Nassau