Thanks a lot for your advices, i will do all as you say and if you want I'll send you the results at the beginning of each season and you will have information of how it went. I had a wrong idea of how i should train them...
I have three possible options for the third trainee, these ones:
(20556365) 22 years old +1 training of JS
Jump Shot: strong    	 Jump Range: mediocre
Outside Def.: mediocre  	 Handling: inept
Driving: average      	 Passing: awful
Inside Shot: mediocre 	 Inside Def.: average
Rebounding: respectable Shot Blocking: inept
Stamina: strong 	         Free Throw: inept  	 
Experience: atrocious 	
(16631594) (this one is the best option i think) +3 training of JS
Jump Shot: respectable 	Jump Range: respectable
Outside Def.: inept 	        Handling: respectable
Driving: pitiful          	Passing: mediocre
Inside Shot: respectable Inside Def.: strong
Rebounding: strong 	Shot Blocking: inept
Stamina: respectable 	Free Throw: average
(17776857) the youngest
Jump Shot: average 	Jump Range: inept
Outside Def.: mediocre 	Handling: respectable
Driving: respectable 	Passing: mediocre
Inside Shot: respectable Inside Def.: mediocre
Rebounding: mediocre 	Shot Blocking: inept
Stamina: awful 	        Free Throw: pitiful  	 
Experience: pitiful 
As you have adviced me i'm training JS for fowards, and i'll keep doing it until the end of the season to get a proficient(as minimum) level for the SF. Then i'll train OD and ID. I started the game one month ago and your advice sounds good. I'll get results in long-term and with lucky maybe i get the promotion. 
If you think that the players exposed before are not good for training I have 262k$ to buy a new trainee or if you think  it's better to sell them for buying an old player with lot of defense.. I don't know xD I think i should buy a PG or a C to get more defense and get the promotion (not for this season, i think it's too complicated) any feedback is good to me.
P.S: I think i'll never have a marvelous JS xDDD Should I train 2 seasons to get it, isn't it?
thanks a lot!