I made this up at a Denny's at 2am, and I was not intoxicated in the slightest:
A successful businessman is walking home from work one evening, when he decides to take a shortcut through the bad section of town. As anyone reasonable person would expect - given that I'm telling a joke and I took the time to mention that "bad part of town" - the man soon finds himself surrounded by three hoodlums.
"Okay, man," the largest thug says. "Just give us your wallet, and nobody needs to die tonight."
The businessman holds up his hands. "Alright, alright!" he says. "I'll give you my wallet! Only... nah, you wouldn't be interested."
"What are you talking about?" another of the muggers asks.
"It's nothing," replies the businessman. "Just that... I have a leprechaun in my pants, and if you let me go, I'll have him grant you three wishes."
The third hoodlum points a gun at the businessman's face. "If you're talking about your dick..." he menacingly begins.
"I'm not!" the businessman interrupts. "It's a real leprechaun! Just... well, he's shy, so you'll need to be really quiet."
The three criminals discuss this for a moment before coming to a decision. "Okay," the largest one says, "we'll take those three wishes."
"Great!" the businessman sighs with relief. He reaches down and starts undoing his zipper, but then hesitates. "Okay, you'll need to get down on your knees in front of me. He'll get scared if you're not eye-level with him."
The muggers glance at each other, shrug, and get down on their knees in front of the businessman's crotch. He starts undoing his zipper again, but pauses once more.
"I just realized," he says, "that if there are three of you, you'll each only get one wish. Is... is that still okay?"
"Man, just get that leprechaun out already!" one of the thugs yells.
"Quiet!" hisses another one. "Don't scare the leprechaun!"
"Thank you," the businessman says softly. "So, one wish each?"
The criminals all nod.
The businessman returns the muggers' nods, then finishes undoing his zipper... and suddenly, he whips out his dong and starts jerking off in their faces.
"Fuck you!" he shouts. "Leprechauns don't exist!"