C3 requires either WebGL 1 or WebGL 2. For the purposes of Construct WebGL 2 is not particularly important; it will use it if it's there but there won't be much meaningful difference in how the game runs. (It might be a little more efficient sometimes, but that's about it.)
WebGL 1 support is effectively ubiquitous these days, being supported on virtually all desktop and mobile devices.
Coincidentally, there's a lot more background in this blog post I recently wrote.