This seems to be "Broken Toys" by Gordon Dykstra (his only published story to date) in Asimov's, June 1980.
"Mister" Aldous, the protagonist is doing a survey of the comatose and brain trauma patients in a hospital. The girl dreaming of a horse with three legs was severely brain damaged in an accident, her frontal lobes mostly destroyed.
What was she thinking? Depends on what you mean by thinking. Sure, she felt pain, of that indefinable something’s-wrong-deep-inside sort, but she didn’t have enough left to ponder the pain with. She was thinking of a horse-a horse with three legs. Even the complete image of the horse was damaged and wavery, like a bad tape spinning around and around a closed loop. It wasn’t pleasant and it certainly wasn’t human.
There's a man who is really brain dead, but still showing a trace on the electroencephalograph so the machines keeping him alive can't be turned off.
As we left the room I took one last dip into the man on the bed. I felt a sense of existence, but nothing more. The part of him that had given him a personality had long since departed.
The last patient Aldous "visits" is a comatose elderly woman who was severely burned in a car accident. She's reliving the highlights of her life in a very short loop, continually re-experiencing the trauma of the accident. Aldous is charmed by the other memories of her life, but can't imagine re-experiencing the accident again after he does it once.
There was that presence in the room, a woman with her life going around and around in her shattered head. As fast as neural circuits could exchange we had shared a lifetime together and she had taken me wholly into that life, as father, brother, lover, husband. She wasn’t scared of me.
I wanted to join her again more than anything else I’d ever done before. Then there was a roar and a hurtling shape and a brief unbelievably intense POW and the memory of the accident crushed that urge forever. I couldn’t go through that again.
But she was. She was going through that memory several times a minute. And the fire after. And the screaming as her skin fried off her bones.
Late that night he slips into the hospital and the story ends just before he unplugs her.
Hello Jamie, love, love, love...
There was just about enough light for me to pick my way behind the clutter of boxes to find the big cable supplying the power. Delicately my fingers picked their way down the line until they touched a knot of metal leading into the wall: the plug, as they say.
I grasped the rope with both hands and prepared to yank, but I hesitated.
