https://codeiq.jp//challenge/1618 with set

solve.rb

#!/usr/bin/env ruby

target = gets.to_i
count = gets.to_i
values = {}
count.times do |i|
  values[gets.to_i] = 1
end

answer = 0
values_a = values.keys.sort

values_a.each_with_index do |vi, i|
  values_a[i.succ, count].each do |vj|
    need = target - vi - vj
    # sortしてるので、`3つめより2つめが大きいか同じ == これ以上先に進んでも満たせる可能性がない`
    break if vj >= need
    answer = answer.succ if values[need]
  end
end
puts answer