That's true, when the spring is broken the gearbox is completely locked, but there's another spring inside the preselector, or something it's stuck between the two wheels of the preselector when they try to reach the "3rd gear position".
If you don't hear other weird noises form the gearbox I think the problem can be located in the preselector, but of course I'm not a mechanic!!
Be careful to don't mess with the forks, spring etc in the preselector, it can be painful!