The Trio is called the Yggdrasil Trio
composed by, as you know, Xerneas, Yveltal, and Zygarde.
Looking into it, it does make sense and has a deep explanation.
The others you mentioned are called the Lake Trio, probably cause you find them in a lake?
...I dunno I didn't play gen 4 ¯_(ツ)_/¯
Here are the links:
Hope this helps buddy!