3 options about the 3 additional nations should be:
Option 1: United Kingdom (because of its big role on the western front), Österreich-Ungarn (as the biggest ally of Germany and the big role in the campaigns in norther Italy) & Italia (people everytime forgetting about the front in Italy)
Option 2: United Kingdom (because of its role on the western front & Gallipoli), Ottoman Empire (Gallipoli and to represent the asian front), Russian Empire (they fought against Germany and Ottoman Empire, so very important here)
Option 3: United Kingdom, USA, Russian Empire (simple: all the major powers of that period would be in. Tough I don't like the fact USA would be in, because they had nearly the same equipment as UK and joined the War really late)
So basically, UK should be in. That's out of the question.