In "The Monkey's Paw," why is Morris so afraid after he makes his third wish upon the monkey's paw?

Unless I am mistaken, you did not mean "Morris" in your question but Mr. White. Sergeant-Major Morris made three wishes while he was in India, but he does not seem afraid in the story, only revolted. And he does not say anything about being afraid after making his last wish while in India. On the other hand, Mr. White is terrified when he makes his third and last wish, and he still seems afraid after...

Unless I am mistaken, you did not mean "Morris" in your question but Mr. White. Sergeant-Major Morris made three wishes while he was in India, but he does not seem afraid in the story, only revolted. And he does not say anything about being afraid after making his last wish while in India. On the other hand, Mr. White is terrified when he makes his third and last wish, and he still seems afraid after it seems as if his wish has been granted. He does not even venture outside after the knocking ceases until his wife goes out. It seems completely understandable that White would be so afraid even after the knocking had stopped. In fact, he could be even more afraid.

The fact that the knocking has stopped does not mean that the thing outside has gone away. The narrator does not tell us what White's third wish was, although we have a good idea without being told. No doubt White wished for two things: (1) for the knocking to stop, and (2) for whoever is doing that knocking to go away forever. But White doesn't have complete faith in the monkey's paw. The horrible creature he imagines--and we imagine!--standing right outside his front door might be cunning as well as hideous. It might have stopped knocking in order to entice the old couple to come out. It might be waiting in silence right beside the front door, ready to pounce when the couple emerges.

We can feel just as frightened by a dead silence as by a loud knocking. We are identified with Mr. White, and we don't like the idea of risking our lives by going out into that dark, cold night with him in our imaginations. Mrs. White is not frightened because she has never been afraid of her own son. And she has never been afraid of him because she never saw him when he was dead. That is why she goes out first. Then, when nothing happens to her, 

...a long loud wail of disappointment and misery from his wife gave him courage to run down to her side, and then to the gate beyond. The street lamp flickering opposite shone on a quiet and deserted road.

The "long loud wail of disappointment and misery" is pathetic, but it tells Mr. White that there is nobody out there, and this is what calms his fears and gives him courage to venture out himself. He goes all the way down to the gate and sees that his last wish has been fully granted. We will never know for sure whether it was Herbert or some stranger who was lost in this dark, isolated suburb and trying to get directions. The fact that there is no one visible on the road suggests that it was more likely Herbert and that he vanished by magic.