I founded the solution but it's weird. In my code, i used only the ">" operator in all my conditions. The last statment couldn't be ok. Then i tried with the ">=" operator. What i thought was : if 2 numbers are equal but superior to the third, then the 2 numbers would appear first as it was requested. Is that right or am i wrong ? Because i wrote my algorythm like that : if number "a" was STRICTLY superior to number "b" and number "b" was STRICTLY superior to number "c", displays "a b c". I thought what was request was to strictly separate numbers that are superiors to inferiors numbers. 
package fr.codegym.task.task04.task0420;

/*
Tri de trois nombres
*/

import java.io.*;
import java.util.Scanner;

public class Solution {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        //System.out.println("Taper a");
        short a = sc.nextShort();
        //System.out.println("Taper b");
        short b = sc.nextShort();
        //System.out.println("Taper c");
        short c = sc.nextShort();

        if (a >= b && b >= c) {
            System.out.println(a+" "+b+" "+c);
        }
            else if (a >= c && c >= b) {
            System.out.println(a+" "+c+" "+b);
        }
            else if (b >= a && a >= c) {
            System.out.println(b+" "+a+" "+c);
        }
            else if (b >= c && c >= a) {
            System.out.println(b+" "+c+" "+a);
        }
            else if (c >= a && a >= b) {
            System.out.println(c+" "+a+" "+b);
        }
             else if (c >= b && b >= a) {
            System.out.println(c+" "+b+" "+a);
        }
    }
}