My Spare Boyfriend (MMF Bi Threesome)

Chelsea has driven Tom to University to start the year as the roommate of her old boyfriend, Mark. He’s nowhere to be seen though, and they’re all alone, so they start getting passionate.  But when Tom comes back from taking a quick shower, he discovers that Mark is back too, and that he and Chelsea are behaving totally inappropriately.  In fact Chelsea wants to show Tom a new way of doing things.  Will Tom be mad, or will he try breaking through some boundaries that he never even thought about crossing before?

Excerpt –

"What the hell?" Tom said, and slammed the door shut behind him.
Nobody said anything at all in reply. Mark's head just kept going, while Chelsea's face was intense and distracted. She extended one arm though, and crooked a come here finger at him.
Tom stared at the two of them, and his face was starting to grow red.
"What the hell?" he repeated. "You're just going to cheat in front of me? That's it?"
Chelsea held her hand open, extended towards him.
"I'm not cheating,” she muttered. "Cheating is sneaking and hiding. I want you to come here."
"What the hell?" he said again, weakly.
"Come over here," she said, "and hurry."
Tom stood watching the two of them silently. His hair was still wet from the shower, and he was wearing only the blue towel wrapped around his waist.
Chelsea looked over at him now, her eyes wide open. She arched her back and waggled her legs in the air.
"Hurry," she said, and held out her hand again.
"The lady wants you to come over to her dude," Mark said. His voice was muffled against her. "You should listen."
Tom shook his head, and walked slowly up to the two of them. He stopped in front of the bed, his leg pressing up against Mark's.
"Finally," she said, and pushed herself up to a half-sitting position. "Kiss me."