I guess you exported your game with Node-Webkit?
So, I think in Construct 2 you see the memory usage only of your game (all the images, sounds), but when
you actually run the game then the run-time (Node-Webkit) needs memory too, because in the end Node-Webkit is a special version of the Chrome browser. And it's not 3 instances of your game running, it's 3 threads of Chrome/Webkit.