Solve the Problem $\frac { 1 } { 210 }$ B) $\frac { 1 } { 343 }$

Solve the problem.
-There are 7 finalists in a singing competition. If a person guesses randomly the top three winners (in any order) , what is the probability that they Will guess correctly?

A) $\frac { 1 } { 210 }$
B) $\frac { 1 } { 343 }$
C) $\frac { 1 } { 35 }$
D) $\frac { 1 } { 21 }$