Middle
Barry likes sleeping in the middle.
Likes the way Eddie's arm feels around his waist, breath tickling the back of Barry's neck. The firm, warm pressure of another body against his back and the sense of being protected it gives him. Spooned up by Eddie, Barry is safe all night long.
But that's not all Barry likes about sleeping in the middle.
There's also Iris, curled against his chest, his arms around her back and fingers feeling the bumps of her spine. The soft rise and fall as she breathes, legs tangling with his, and with Eddie's, in the night. She makes him feel safe too, but it's a different sort of safe than Eddie gives him.
Eddie makes Barry feel like he doesn't have to be the hero twenty-four-seven. He can relax and let someone else protect him. Iris makes Barry feel like he never has to be less than himself; he can be Barry Allen and the Flash and they'll both still love him just the same.
So Barry likes to sleep in the middle, in that safety net his lovers make for him. Where his nightmares melt away.