All of these languages were very similar to each other more than 100 years ago but were actively forced to diverge for political reasons during the turbulent 20th century.
Czech/Slovak/Polish
Macedonian/Serbian/Bulgarian
Croatian/Slovenian
Of course outliers like Romanian/Greek/Turkish/Albanian/Hungarian are different by construction. But even they shared extensive vocabulary with the rest due to the free flow of (rich) people between the countries.
All of these languages were very similar to each other more than 100 years ago but were actively forced to diverge for political reasons during the turbulent 20th century.
Czech/Slovak/Polish
Macedonian/Serbian/Bulgarian
Croatian/Slovenian
Of course outliers like Romanian/Greek/Turkish/Albanian/Hungarian are different by construction. But even they shared extensive vocabulary with the rest due to the free flow of (rich) people between the countries.