Depends on where in those countries you are going, where you will stay, and what you will do.
Airfare to all 3 places is about the same - $850 - 1200 USD average. Hotels in the big cities are all about the same price in US dollars. If you go to smaller cities to stay, it is cheaper.
The economies of all 3 countries are not the best, and the US dollar goes a long way. Cities like Moscow or St. Pete compare in price to Kyiv or Minsk.
I would simply pick the country that has things in it that will interest you. They are all 3 beautiful countries, with many non-Western sights that will intrigue you.
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