One seventh won the first prize, one sixth won the second prize and one third won the third prize.

So the number of participants must be a multiple of 7, a multiple of 6 and a multiple of 3.

The minimum common multiple of 6 and 7 is 42, and the maximum common multiple is 84.

A multiple of 6 must be a multiple of 3.

And less than 50, so 42 people.