So France, Russia, and the UK vs Germany, Austria-Hungary, and Italy. Purely theoretical (I don't think London would be able to stomach signing the sort of hair-trigger alliance this would require), assuming 1914 levels of preparation, that the dispositions of neutral parties are similar (US inclined to respect Entente blockades, Austria has to keep some forces posted to its southern border to guard against Serbian ambitions, Turkey treaty bound to keep the straits open despite its dislike for Russia), and that all participants are in the process of mobilization durring the commencement of hostilities.
In the Triple Entente's favour the Dardanelles are open, they have fewer fronts without the need to fight the Ottomans and Bulgaria, and Britain is immediately in the war.
In the Triple Alliance's favour Italy is an ally rather than an enemy (a second major front for France), and there is no Balkans front.
So, who wins? If it's too one-sided how long does the war last? Would including Romania in the Triple Alliance make a significant difference to the course of the war?