I use this commands in Singleplayer and in (open to LAN)
/scoreboard objectives add deaths deathCount Player Deaths

/scoreboard players add @p deaths 3

so i use a commandblock with:

testfor @p[[score_deaths_min=3]]

the two last commandblock have:
/scoreboard players remove @p deaths 3
/give @p 276

i dont know is a feature or a bug,.

you can see on a picture i press a button a commanblock see i have min 3 deaths, give energie on comparator this take a redstone torch off

a two redstone line can update the last commanblocks.

give me a item 276(dia swort)
and remove me 3 deaths.

So i have 0!! deaths when i use a button now, a command block see i have not min 3 deaths BUT the command block ignore this and give energie on a comparator oO.

when i use playerKillCount, totalKillCount, dummy,. a commandblock dont give energie on comperator.

EDIT:

Mh LOL i testing again the problem is, when i use
testfor @p[[score_deaths_min=1]] comandblock ignore a min level
testfor @p[[score_deaths_min=2]] comandblock ignore a min level
testfor @p[[score_deaths_min=3]] comandblock ignore a min level

and when i use
testfor @p[[score_deaths_min=4]] comandblock don't ignore a min level o.O

and i can't set negativ argoments "testfor @p[[score_deaths_min=0]]" or lower.
i have -5 deaths and "testfor @p[[score_deaths_min=-4]]" dont work
i have -5 deaths and "testfor @p[[score_deaths_min=-6]]" dont work

EDIT2:

So when i leave a singleplayer world and start this world again,. then a problem with a commandblock is away.

testfor @p[[score_deaths_min=1]] comandblock don't ignore a min level
testfor @p[[score_deaths_min=2]] comandblock don't ignore a min level
testfor @p[[score_deaths_min=3]] comandblock don't ignore a min level

and i can set negativ argoments, but i must leav and rejoin the world