Here's link to the answer:

40 players multiplied by 30 teams = 1,200 players in the MLB
53 players multiplied by 32 teams = 1,696 players in the NFL.
1,696 - 1,200 = 496
There are 496 more players in the NFL then in the MLB.
In the first 10 -> 10 - 9 = 1 (contain a 3).
In the first 100 -> 100 - 9 * 9 = 19 (contain a 3).
In the first 10^n ->
10^n - 9^n (contain a 3).
<u>The answer is 3.439 numbers contain a 3 in the first 10.000</u>