I guess I somewhat second Kickman.
Three sport stars are going by the wayside because of various issues. However you cannot overlook certain requirements to play a sport.
1- any sport you must be an athlete.
2- to play american football you must be a certain size
3- to play basketball you must be a certain size
Therefore the 3 sport star (which by the way is ussually referred to as Football/basketball/baseball) can only particpate if he has the athleticism and the size.
Some people I worked with have sons who played football...right height, wrong weight or speed or whatever. Problem is the kid went to all the practices and in 3 years of HS played a total of 15 minutes of game time...one quarter(and I understand some did not even play that much).
He quit...
Some sports are contradictory.... i.e. a swimmer must be long & lean (long muscles) and a wrestler must be powerful (short muscles) so you can't do both.
So the choices are limited to 95% of the population.
The playing of games is predicated on how serious an athlete is about the game... what are the longer term expectations?.. If I am 5'9" and 155lbs and would like to play a sport after HS my choices are limited.
Throw in demographics and I think you will find that athletes will gravitate to the sport they are most succesful in. Coaches will then make the final decision for him/her based on his "commitment" to the sport
All this to say this is an extremely small % of the population today and would guess (although bigger) was a small % back then.
So what is all the fuss about? As Hurst said let them try when they are young and see what they pick. Problem is you can't really play football and basketball when you are little because that is not the real game
BTW I was a 2 sport athlete in HS...