Switzerland is famous for being made up of three language groups (German, French and Italian). When, how and why did these groups decide to confederate into a single nation? How was a sense of Swiss identity formed? Did inter-ethnic tensions ever lead to mass violence?

Well, it should first be said that it depends on the different cantons. The French-speaking canton of Vaud, for example, first "joined" Switzerland by being conquered by the canton of Bern from Savoy in 1536, winning independence first when the Helvetic Republic was created in 1798, which was confirmed at the end of the republican experiment when it became one of the cantons of the 1803 Confederacy. The canton of Jura was also part of Bern for a long time, and only became independent in 1979. Bern still has a sizeable francophone minority! Italian-speaking Ticino has a similar history, being conquered during the Swiss expeditions against the French in Italy in the 15th and 16th centuries and administered jointly by the cantons as a common bailiwick, becoming independent at the same time as Vaud. I should point out that this did not only happen to non-German-speaking regions, as the same thing was true of cantons like Thurgau.

On the other hand, cantons such as Geneva, Neuchâtel or Valais were separate, independent states which were allied to the Swiss confederacy and ended up joining it proper in 1814-1815 (Though Geneva and Valais suffered some trouble and were annexed by France during the Napoleonic era). Fribourg, while also German-speaking in part, did the same much earlier, joining up with the Swiss when threatened by the duke of Burgundy Charles the Bold in the 15th century. It's interesting to note that this is when Fribourg also started to use German much more widely as an official language: Switzerland was not institutionally multilingual until the 19th century, when all the French speaking cantons joined the Confederacy.

There was never really any mass violence between the cantons on ethnic or linguistic grounds: most civil conflict happened on religious lines, either during the Reformation, or, more recently, during the Sonderbund war in 1847. Swiss identity itself came to being during the 19th century, with the creation of Switzerland as we know it today: it relied heavily on the shared history between the cantons, a sense of tradition fostered by the creation of patriotic events like the Unspunnenfest, and a shared political system and will for independence. There still also remained a deep attachment to ones' canton, though it is increasingly less true nowadays.

If you can read French, German or Italian, the Swiss Historical Dictionary has good articles with a bibliography on the individual cantons, languages and even on plurilinguism and the idea of the nation itself. My sources in English are lacking, but in French I'd direct you either towards the older Nouvelle Histoire de la Suisse et des Suisses or François Walter's Histoire de la Suisse if you want a more general history of Switzerland.