I’d look for flights to/from Ukraine. Kiev is a long ways from anywhere else in your itinerary plus Ukrainian trains use “Russian gauge” so there’s time spent at the border switching wheels or switching trains. The roads in Ukraine are awful (although the ones in the western part near Kiev may be slightly better than those in the east where I stayed) which makes any trip by bus a long nightmare. I’d use www.whichbudget.com using Kiev as your departure point to see which two of your other cities has the best cheap flight options.