Решил! Но совершенно не понимаю почему оно работает.

public class Solution {
    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        int a = Integer.parseInt(reader.readLine());
        int b = Integer.parseInt(reader.readLine());
        int c = Integer.parseInt(reader.readLine());

        if (a < b) {
            int temp = a;
            a = b;
            b = temp;
        }

        if (b < c) {
            int temp = b;
            b = c;
            c = temp;
        }

        if (a < b) {
            int temp = a;
            a = b;
            b = temp;
        }

        System.out.println(a + " " + b + " " + c);
    }
}