Just so you know Wii boxing is indeed not a 3D space game. Everyone knows it bases it stances off of controller orientation.
Now the Wii remote can indeed find itself in 3D space using it's current technology based on simple trig.
It can do this by figuring the distance it sees the two dots apart using that with the pre-set distance of the dots on the sensor bar. Using that it can figure the angle between the two dots at it's point. from there some simple trig will tell it it's possible distance from the bar thus you have the first spacial dimension. (Red Steel has an example of this with it's "zooming" feature)
Then depending upon the angle that it is pointing at in the vertical direction it can then again utilize some simple trig to find its height which is the second dimension.
Finally depending upon how far to the side the wii-remote seems the two dots it can determine it's horizontal position.
There are the three dimensions. The only issue is that to find it's actual position would need alot of processing power and that would significantly slow down the system.