Oh dear.
For one thing, there is no piston in a Wankel engine. A piston is a component of a reciprocating engine, a.k.a. a piston engine. A Wankel engine is a rotary engine, which is a completely different animal from a piston engine and is also known as a pistonless engine.
What a Wankel engine has is a rotor.
And for another thing: can the rotor really be described as triangular? A triangle is a geometric figure, and surely has to have straight sides; the three sides of the Wankel rotor are curved. Wikipedia compares the rotor to something called a Reuleaux triangle, which it describes as a "curved triangle". (The Reuleaux triangle, like the 50 pence piece, has a constant diameter; its sides are actually more curved than those of the Wankel rotor.)
If the question had asked how many sides the Wankel rotor had, I wouldn't have argued – except maybe to point out that the sides were curved.
Wikipedia has an interesting moving diagram of a Wankel engine in action.
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