Wildsky, sorry...didn't realize my post was so late after everyone else's. You can tell which of the three tabs is the positive connection because you can see it connected to center of the bottom of the switch (if you put the switch face down with the tabs sticking up at you).
As for the other two tabs, you can choose to connect to either one, but you must still create a "bridge" to make continuity from those tabs to the actual threads of the switch, which acts as your negative.
To do this, orient the switch so the positive tab is at your 12 o'clock position. At your 9 o'clock position, you'll notice a piece of metal running up the side of the switch and under the bezel ring. This is where you need to create your "bridge". So, what I did (taken from Apo's post) was melt solder at the point where the metal runs under the metal bezel ring. This creates a connection between the threads of the switch and your other remaining negative tab. Now you should have not problems w/ your connections!
I also chose to break off one of the unneccessary negative tabs. Which one you choose to break off is immaterial, but I always break off the one at the 6 o'clock position. I also take that broken off tab and run it under the bezel ring where I make my solder bridge so that I'm using less solder to make the connection.