In order to fulfill this outcome:
The unification of the German Empire under Prussian leadership occurs according to the Großdeutschland solution, including the 1870 territories plus Austria, Bohemia, Moravia, Austrian Silesia, and Slovenia;
The unification of the Kingdom of Italy under Sardinia-Piedmont leadership occurs including the 1870 territories plus Trento (not necessarily Bozen), Gorizia-Gradisca, Istria, and Dalmatia;
An independent Kingdom of Hungary is established, which includes the 1919 territories plus Slovakia, Transylvania, and Croatia (minus Dalmatia);
the three states are allies and fight the WWI together;
which PoD do you deem most plausible, 1848 or 1866 ??
In order to place these kinds of events in a broader TL, you may also assume that:
The USA have been able to conquer Canada during the ARW, the French Caribbean, British West Indies, and Australia, during the Quasi-War and the War of 1812, Spanish South America (minus Chile and Argentina) during the Wars of Independence in SSA, and Mexico during the Mexican-American War.
(possibly) Belgium gets partitioned as a result of the Belgian Revolution, with Flanders going to the Netherlands, West Walloonia to France, and
the parts of the provinces of Liege, Limburg, and of Namur east of the Meuse river as well as the Grand Duchy of Luxemburg, to Prussia.
The Union wins the ACW against a Confederacy-British-French alliance.
Soon after the unifications of Germany and Italy, France loses a war against the German-Italian alliance.
The USA-Germany-Italy-Hungary Quadruple Alliance wins WWI against the Britain-France-Russia Entente.
Now, I've tried to puzzle out proper PoDs for the unification events. As it concerns 1866, I assume that the Austrian Empire suffers crushing defeats both at Sadowa and Custoza, its army falls into disarray, Hungary rises in open rebellion, Prussia and Italy advance in the collapsed Empire and partition it with Hungarian nationalists. France is distracted by the ACW, Russia is busy suppressing a Polish revolt and is mollified with the cession of Austrian Poland and Galicia. In the face of the collapse of the Hapsburg, minor German states accept union in the German Empire under the leadership of Prussia.
As it concerns 1848, things are more difficult to put into proper place (also because I'm less familiar with the history 1848 revolutions in Germany, Austria, and Hungary).
I very tentatively assume that Russia is kept busy by a Polish revolt, France is paralyzed by its own revolution, the Haspburg army suffers several defeats in the war with Sardinia-Piedmont, Austrian liberals rise up and Hungary manages to break free completely from the Hapsburg Empire, which falls into chaos, the Frankfurt Parliament implements the Großdeutschland solution and offers the crown of the German Empire to the Prussian King Frederick Wilhelm IV, who is either persuaded to accept or abdicates in favor of his brother Wilhelm I. In the face of Hapsburg chaos and Russian repression, Czechs and Prussian Poles agree to be part of Germany for minority rights (schools and laws in their own languages and German). Sardinia-Piedmont is able to seize Italian areas from the collapse of the Hapsburg Empire, gaining much prestige, revolutions sweep the minor Italian states, and the Kingdom of Italy is proclaimed under the leadership of the Savoia.
In both PoD, after a few years, France sufficiently recovers from internal unrest or defeat in the ACW and tries to redress the balance of power in Europe by going to war with Germany, but it is defeated and forced to cede Alsace-Lorraine and Luxemburg to Germany, and Savoy, Nice, and Corsica to Italy.
Opinions ? Suggestions ?