Maybe the third one would be YO :P jk^ Ever since RSE yes. But how could Yin-Yang have 3 when it is 2.
Reshiram uses a natural flame which moves the atmosphere
Zekrom uses a generator (rather well known for being man made).
I'm still seeing an Industrial vs Natural theme, if only somewhat.
Nature and technology may be opposing forces, but they're not balanced in the way of Yin Yang. They don't support each other, you can have one without the other, etc.
^ Ever since RSE yes. But how could Yin-Yang have 3 when it is 2.