There's the damage triangle, Tanks beat Burst, Burst beats DPS, DPS beats Burst
There's the support triangle, sustain, poke, hard engage. I believe the triangle goes sustain beats poke and then so on, but this triangle is more flexible, or at least more skill dependent.
There's the Jungle triangle, where farm Junglers don't care about control Junglers, control Junglers are a bitch towards ganking Junglers, and ganking jungles accelerate the game too much for farm Junglers, however this is play style dependent moreso than champion dependent if you really know what you're doing.
There are triangles everywhere. Some more subtle or skill based or play style based or whatever than others, but they're typically there.