They come in Threes
Bad things come in threes is an old superstition. People use it to confirm that nothing else bad will happen in their lives when something goes awry. The Monkey's Paw iself is based in old superstition (be careful what you wish for you, you just might get it). I find it charming that the paw grants three wishes and that it was made for three men.
"It had a spell put on it by an old Fakir," said the Sargent-Major, "a very holy man. He wanted to show that fate ruled people's lifes, and that those who interefered with it did so to their sorrow. He put a spell on it so that three separate men could each have three wishes from it."
Morris mentions that the first man used his final wish for death and that is precisely how he got the monkey's paw. That makes him man number two. Mr. White becomes the third eventually wishing away the evil that is coming towards them. I find it particulary interesting that Western Culture has two specific ideologies about the number three. The first is that bad things come in threes, but we also say "Third time's a charm" to put hope and good energy into a third attempt at something. This means that although two failed attempts caused trouble or issues, the third should be enough to discount all the bad from the first two tries. But by our logic that bad things come in threes, wouldn't it be more logical to believe that the third time would also fail (that is something bad happens three times). Intrestingly enough, the story itself is broken into three parts.
Unfortunately, we do not know what Morris wished for. But we do know that the Mr. White's first wish led to the Herbert's death. Mr. White then makes his second wish the terrifying revival of his son at his wife's behest. His third wish sends "Hebert" away as way to protect Mrs. White. Although it could be considered "good" the last wish finalizes the White's grief over their lost son. Perhaps this in itself is a bit of "third time's a charm" as Mr. White finally wishes for something sensible. The Whites must live with the wishes for the rest of their lives. In part this grief is made worse by the idea of having caused his death by making the initial wish.
The feelings of grief and loneliness are solidified by the flickering streetlamp and the deserted road. Hope is fleeting, flickering maybe disappearing on an empty road. On the other hand, that flicker could mean that hope still exists despite all of the empty loneliness that will grip the Whites.
I do not know that bad things necessarily come in threes or that the third time is a charm. But I am drawn to the common number of three within the story. Three men, three wishes, broken into three parts. Considering old superstitions, I find it fascinating that the repitition of three plays towards an old idea.