A common mountain range covering Bulgaria and Serbia is called the Balkan Mountains (in Bulgarian also called 'Stara Planina')
Its the Danube river and Danube plain separates Romania and Bulgaria. There are no mountaneous border between the mentioned two countries. And Carpathian mountain range of Romnia or Balkan mountain range (most northern mountains of Bulgaria: none of them serves as border.
The Balkan Mountain range (in Bulgaria Balkan Mountain is often called the 'Stara Planina' Mountains)
Bulgaria.
Serbia
Hungary, Serbia, Bulgaria, Moldova, Ukraine (and the Black Sea).
If you do not consider Kosovo to be a part of Serbia, then the answer is: Macedonia and Bulgaria. If you do consider Kosovo to be a part of Serbia, the answer is: Albania, Macedonia, and Bulgaria.
Macedonia shares its borders with Serbia, Albania, Greece and Bulgaria
Serbia
The Balkan mountain range runs from the border with Serbia 560km eastwards to the Black Sea
Surrounding countries are Ukraine, Hungary, Serbia, Bulgaria and Moldova.
Romania borders Bulgaria, Serbia, Hungary, the Ukraine, and Moldova.