Tributary seems to be an intruder here, it's the only one which does not begin with the prefix meaning "three". Trifecta would be equivalent to "tiercé"? It's not in my dictionary.
But it's not about the "meaning" of the prefix; the quiz asks for those words beginning with "tri". I doesn't even require it to be a prefix technically, although most of the answers are.
Couldn't a triad be a group of three leaders? And aren't pitchforks and tridents different things? Pitchforks (usually four-pronged) are for hay, tridents for fish.
The spelling of arthropod still needs to be corrected. This might seem like a minor issue, but as a nit-picker I'd like to pick arthropods and not athropods.
But seriously, no "triforce"?