The all have something to do with trees. The first one is read き (ki), and it means “tree“. The second one is read はやし (hayashi), and it means “woods“. The last one is read もり (mori), and it means “forest“.
So basically, the more trees you add to the kanji, the “bigger” the concept becomes! Unfortunately there is no kanji with four trees 🙂
森林 (shinrin) can also be written, having roughly the same meaning, “woods”, “forest”, or “woodlands”.