I think you'll find the answer is in fact 15.
Once you get those three passed the electric wall there are 12 more depressed mudokons in some tiny cells that the first three help you save.
If it's any consolation MM, it means you're no more of a loser than Ench, who also got it wrong, and no less of a loser than Carnix, who hasn't yet learnt the difference between 'loser' (person who loses) and 'looser' (person who sets things loose).
Also, I apologise that I am not my usual cheery, PC self in this post.
I have a cactus in my throat, a boulder in my head and a jellyfish up each nostril, therefore demonstrating that
even though I may know the right answer to an obscure piece of trivia about a computer game that no one's played for 10 years, I'm a loser, too.