What is the difference among a canyon, gorge, and valley?

The easiest way to answer this is to describe each term, so you can see:

1) A canyon is a deep valley with steep sides thinks of the Grand Canyon most famously to imagine what this is.

2) A gorge is a deep ravine, which generally has a river running by

3) At last, a valley is any depression, generally of a certain length, in the surface of the land and often having a river.

Thus, a canyon is a exact type of valley, with particularly steep sides. A gorge is deep depressions that will usually having a river.