The Dutch were the first to use the tricolour during their revolution, making themselves a republic. The tricolour then became a symbol of republicanism and became very popular during the French Revolution which shook the ground of Europe, inspiring republicanism in other countries to use the tricolour, for example the Irish tricolour during the easter rising.
Another interesting fact is that the Russian flag was a tricolour when it was an absolute monarchy, it was because Peter the Great was inspired by Dutch ships to create a modern navy, he adapted the Dutch flag for the Russian navy since the Russian navy didn't have a flag at the time. The flag inspired the flag of pan-slavism which is why eastern european countries have those three colours in their flags (Slovakia for example) then became a symbols of Russia and made the national flag in 1894. Which is why eastern Euroean countries have a similar tricolour.