Basically the tricolor often symbolized revolutions and liberty, and generally Republican values as well. The first example still in use today is the Dutch "prince's flag" under which Dutch rebels were fighting against Spanish rule.

But the tricolor as a symbol of Freedom and Republic really caught on with the French Revolution, when the revolutionary militias of Paris began to use blue and red flags (blue and red being the traditional colors associated to Paris, still today) and added the white that was considered the traditional colour associated with France (I guess it will make some ignorant people on Reddit smirk but the significance had nothing to do with cowardice, it was simply the color of purity).

The design then spread in Europe when Italian Republics formed under the rule of Napoleon and adopted a flag similar to the French one to signify freedom from Austrian rule.

A lot of country liberating themselves from oppression whether from a foreign rule or from a monarch then followed the trend of tricolors as a symbol of freedom, with some exceptions such as Russian flag that dates from the Tsarist era and therefore had nothing to do with freedom at all. :)

You can check the very nicely made "Flags through the ages and cross the world" by Whitney Smith if you want a more in depth analysis.

"Les emblèmes de la République" by Bernard Richard focuses specifically on the French symbols but I am not sure whether it exists in English or not.