Run 3 is definitely the most challenging
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Run 3 takes place in a series of tunnels in space. There are ten playable characters, each with their own personalities and abilities. There are currently more than 20 playable side tunnels, and 309 levels currently (367 including minigames). Run 3 is definitely the most challenging and unique game in the Run trilogy.
There are two main game modes. Explore mode and Infinite mode. There are also some other game features, such as the Shop and the Level Editor, while there are also thousands of other levels published by other players made with the Level Editor.
Several characters can be unlocked by playing through Explore mode, as well as the winter and Halloween costumes. The Runner's galaxy map acts as a level hub for Explore mode, displaying all the tunnels and levels that can be explored.