Credit: Denise Truscello/

And then there was one.

Two of Hugh Hefner's three girlfriends -- 20-year-old twins Kristina and Karissa Shannon -- are moving out of the mansion.

The Playboy founder, 83, tells E! News that his relationship with girlfriend Crystal Harris, 23, has been getting "more serious" and that the twins have expressed a lot of interest in doing more Playmate-related promotional work for the company. So they're moving into the neighboring Playmate House.

"They've been hanging out with a lot of the other Playmates and they see these girls traveling across the country to do promotion," he says. "They would like to be doing that, too."

The move -- which Hefner stressed will not affect their E! Show Girls Next Door -- will take place within the next couple of weeks.

"They thought it was a good idea," he said after he suggested the move to them. "They will now be friends, but they won't be defined as the girlfriends."

Now that he has Harris all to himself, will wedding bells be next?

"I haven't had a lot of luck with marriage," he says. "I don't have a good track record, and I don't want to screw this one up."