First era: Slavs, Scythians, Dacians or Thracians
2nd era: Kievan Rus or/and Novgorod, Bohemia or Great Moravia, Bulgaria
3rd era: Poland, Czechoslovakia, Ukraine
Slavs - to serve as progenitor for Slavic peoples, also very cool entity
Scythians - to serve as an acceptable progenitor for Poland, Rus, Hungary, Bulgaria and Romania, also awesome Asian civ anyway
Kievan Rus or Novgorod - trying to figure out non controversial second era progenitor for both Russia and Ukraine (Muscovy is obviously off for this reason

besides being boring Moscow->
Moscow continuity)
(alternately we could have both, seeing as there would be enough material to make them very distinctive)
Great Moravia/Bohemia - we've had both Poland and Hungary, time for the remaining massively important Visegrad country; also Poles would accept it as a bridge towards Poland due to a) General very high sympathy towards Czechia, b) It being our closest relative and c) It sort of making sense through Czech role in the later foundation of Polish civilization
Bulgaria - very interesting and very impressive second era coming from Scythians/Thracians/Slavs and turning into Russia/Ukraine (or Romania)!
Poland - in the modern era but going Mughal way of including 17th century as well, mostly because a) it still had a lot of glorious moments despite not being golden era, b) It would be a lot of fresh air after two civs of almost entirely 14th century Poland, c) I still think there is no modern era descendant of Poland that would feel good for Poles
Ukraine - I mean mainly the focus on 17th-18th century Ukrainian Cossack civilization which was semi independent, plus some Ukrainian history of the 19th and 20th centuries. Very interesting and distinctive culture, which deserves spotlight and pop education.
Was not sure about the third ancient era civ - maybe Dacians or Thracians?