\frac{3\dbinom{17}{3} + \dbinom{16}{3} + 17 \times 17 \times 16}{\dbinom{50}{3}} = \frac{3 \times 680 + 560 + 4568}{19600} = \frac{2040 + 560 + 4568}{19600} = \frac{6168}{19600} = \boxed{\dfrac{1542}{4900}}