1. You don't use opengl code yourself, the library does the calls for you. SDL can let you directly write opengl code when you include it though.
2. OpenGL = renderer, SDL = game development library, SFML = game development library.
3. SDL2 has essentially all the features that SFML has, and a bit more. When SFML came out, SDL1.2 had a bad reputation for almost no hardware rendering, no rotation, old, and some other features that it lacked (but there were workarounds), and now SDL2 is above SFML.