\[C_{13}^{3} = \frac{13!}{3!(13 - 3)!} =\]
\[= \frac{13 \bullet 12 \bullet 11 \bullet 10!}{3 \bullet 2 \bullet 10!} =\]
\[= 13 \bullet 2 \bullet 11 = 286\ аккордов.\]
\[Ответ:\ \ 286.\]