I can only imagine what an anxious time you and your wife must be experiencing. I don't know that anyone can say exactly why this pregnancy is going so differently than the twin pregnancy except to say that triplets put even more stress on the body than twins. Your doctors may have a better explanation. It sounds like your wife's doctors are doing all they can, and every day that goes by, your babies will be that much bigger and stronger.
I have one suggestion. I don't know that it will help, but it won't do any harm as it is just a visualization. She can do this on her own, or you can talk her through it. If you do the talking, keep your voice low and gentle. Whichever of you is doing it, don't rush. Take the time to experience each part of it with all the senses .
Have your wife find a quiet time when she will not be interrupted. She should get as comfortable as she can. She begins by focusing on taking easy, slow breaths and as she relaxes, she allows her eyes to close. When she is ready, the visualization begins by imagining that she is in her bedroom and that everything is just the way she wants it. She is warm, and comfortable, and cozy. When she is ready, she goes to the closet, and inside are stairs leading down. She descends the stairs, and at the bottom, is a beautiful lake beach with a little rowboat drawn up on the sand. The waves are lapping quietly on the shore. In the middle of the lake is an island and in the middle of the island is a magnificent tree that shades it. Your babies are living on this island, and the tree provides them with all the good things that they need for the present. Your wife gets in the boat and rows out to the island. Once there, she gets out of the boat and goes to greet your children. She can imagine herself playing with them, cuddling them, whatever she would like to do. She tells them, though, that it is not yet time for them to cross over the lake with her. For now, they should stay safe on the island where the tree will provide them with what they need to grow big and strong. When she feels ready, she can tell them goodbye and that she will come back to visit them very soon (which she can do whenever she wants by repeating the visualization). Then she gets back in the boat, rows back to the shore, and returns up the stairs into her bedroom. To end the visualization, she allows herself to become aware of her surroundings, and when she is ready, she opens her eyes and stretches.
Visualization can be powerful, so it might help, and, as I said, it certainly won't do any harm. At the very least, it might help your wife feel calmer and more relaxed at least for a time.
-- Henci