Still in snapshot 16w39a, confirmed
The redstone clock is too close to the command block and isn't receiving a redstone pulse
I am officially a genius... Ok so I found the issue and it was in fact NOT the command block at all. So see what I had done was used a comparator clock and since the command block was too close to the redstone, it wasn't actually receiving a redstone pulse. I suppose that this can be considered a bug aswell though... I'll put another snapshot of what I mean.
I did try it. That was the first thing I did to see if the command worked. It didn't... That's when I realized it was bugged.
Notice the "Deaths = 6" scoreboard on the side of my screen?
Maybe this snapshot will help you get a general idea of what I'm trying to explain.
I have added the scoreboard into the game with those stats, however the command is still not recognizing that I have 3 deaths. The exact command I used is
testfor @p[score_deaths_min=3]
. I am aware that you need to have 3 deaths and that it tests for the nearest player and I have accomplished all of those listed criteria but it still does not output a signal.
Confirmed in 1.16.1 release.