The issue of influence in the Balkans will be very tricky, but here is something that can help:
- Serbia had a pro-Austrian dynasty in the House of Obrenovic before their defeat in the Serbo-Bulgarian War of 1885, which saw the Karadjordjevic family take power in a coup. Said family who took power were pro-Russian.
- Bulgaria was liberated IOTL by Russia, and to this day many Bulgarians still see Alexander II as one of the two liberators of Bulgaria (the other being Januarius MacGahan, an American journalist)
What Austria-Hungary and Russia need to do is to state which Balkan states will be their client states. If the House of Obrenovic maintains power, then Serbia would probably slide into the Austrian camp, and Bulgaria into the Russian camp. Romania, I am not sure since they want territories that are currently administered by Austria-Hungary (Transylvania) and Russia (Bessarabia), so I'm guessing that the Romanians might lean towards the anti-Three Emperors' Alliance faction. Albania on the other hand, will be very tricky as no one knows which nation will claim them as a client state. (Assuming that Italy will gobble them up if no one else is interested) Greece will definitely be the wild card as no one also knows which side they will take.