The phrase "a triangular piece" is correct and usable in written English.
You can use it when describing an object or shape that has three sides and is in the form of a piece or segment.
Example: "She cut the cake into a triangular piece for each guest."
Alternatives: "a triangular section" or "a triangular fragment".
It came into fashion as something really modest, a triangular piece of fabric.
A round piece of chocolate will taste softer and creamier than a triangular piece, despite the recipe being exactly the same.
For 5 summers, the writer has rented a small, one-story shingled house in Weeset Point, a triangular piece of… OUR FOOTLOOSE CORRESPONDENTS about a gala held in London by the Halley's Comet Society.
When used for divination, the kobdas was beaten with a T- or Y-shaped hammer made of reindeer antler, which caused a triangular piece of bone or metal called an arpa to move along the surface of the drum.
Success has been achieved with a surgical procedure in which a triangular piece of the ITB is resected from the area overlying the lateral femoral epicondyle when the knee is in the critical position of 30° flexion.
On her diagram, she showed how she had removed a triangular piece of flesh, maintaining a margin around the tumour so that the biopsy could check for any stray cancer cells in the surrounding tissue.
On a dusty shelf, hidden behind a religious statue, they found a tiny triangular piece of cardboard with traces of copper, lead, and aluminum on it in exactly the proportions in which those elements are present in the copper wires that are attached to a popular type of Canadian-made detonator.
Most commonly, a small, triangular piece will chip off, leaving a sloped surface where the notch should be.
A curled triangular piece of plastic walks across the bottom of a petri dish as muscle tissue repeatedly contracts, and another triangular sheet, with a different arrangement of heart-muscle cells, mimics the movement of a fish's tail to swim through a solution.
Foley Square is a plaza that is part of Thomas Paine Park, a city park that includes Foley Square, an area filled with trees to the north and a little triangular piece across Centre Street in front of the federal courthouse.
Mr. Ghandour said he didn't notice the error until the room's hardwood floors had been laid down and there was a gap that required an awkward triangular piece to fill.
