So, let's cover alternate type combinations, then:
- Fire/Water/Grass- classic.
- Fighting/Dark/Psychic- Thief, Mage, Fighter triangle. Not mechanic, but thematic.
- Ice/Steel/Rock- Golem triangle. Somewhat mechanic, with Steel being a sub-leader.
- Ground/Water/Flying- Geosphere triangle. Weather trio is this.
- Fire/Ice/Electric-Legendary birds and Tao trio
- Grass/Rock/Steel(/Water)- Muskedeers.
- Electric/Flying/Ground-Kami trio
- Steel/Water/Ghost-Creation trio
- Dragon/Dark/Fairy-Kalos trio.
- Fire/Water/Electric-First Eeveelutions and Legendary beasts
Other notable stuff:
-DP starters had secondary typings that gave them advantage over their enemies. Fighting over Steel, Ground over Fire, Steel over