ajwood said:
...Too bright of a tail light and others could mistake it for brake lights.
On the other hand, having a light that is dark except during braking, a "brake only light," can prevent just such confusion.
The blown fuse was likely the result of running too much current through a fuse designed to handle only two brake lights.
You need to compute the combined draw of all three lights, and size the fuse accordingly; making sure that you don't overload the wiring itself in the process.
Another, though more complicated, method would be to have the brake switch trigger a relay, and have the new light wired directly (through a new in-line fuse) to the battery through the relay.
Where, exactly, did you mount the third light?