Can they be split?
Taking root cuttings would be the easiest and recommended method of propagation.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-propagation.htm
My yellow dipladenia has some long skinny tendrils on it. What are they? They have "bloomed" and have fuzzy things coming off them like a dandelion.
These are very closely related to mandevilla and will act like a vine with tendrils when the light is too low for it too thrive. This is a sign that it needs more light.
As for what you are describing at the ends of the tendrils... I can't be sure since it does not sound like a part of the plant itself. I would really need a photo to identify the issue. It sounds like a fungal infection of the tendril, which is common. I really would need a photo to confirm this, though. No need to stress the plant out even further with an unnecessary treatment if it doesn't have an infection.
Please include a photo, and I will be more than glad to help.
They are all fairly close together in pots infull sun in Summer. The white one grows like topsy the other two shades of pink. Much smaller. Flower until January. Should I prune then. Can the wind effect size of growth.
It is much more likely that you have both dipladenia and mandevilla. Sometimes they can be hard to tell apart but are often mislabeled and sold that way.
This article will give you an idea of the differences and the care of these: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm
I have quite a large Depladenia in a larger pot. I was wondering if there was any way of splitting the plant I already have into two smaller plants for two different pots? I adopted this plant from a family member that didn't want to care for it anymore and I am not sure if this would kill it or be good for it.
No, splitting the plant in not recommended.
This article will help with propagation.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-propagation.htm
What is the best fertilizer for dipladenias?
Feed potted mandevillas with a ready-to-use, slow-release 18-6-12 fertilizer every three months while they're actively growing.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm
My husband bought me a Rio Dipladenia,when I was transferring it to a bigger flowering pot 2 of the stems with buds broke off. Is it possible to put the stems in soil and have root regrowth ?
You can propagate from cuttings.
The link below though it states Mandevilla is exactly the same for a Dipladenia.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-propagation.htm
Only a few leaves had turned yellow. All others looked perfectly fine.
Since you say it's new, I imagine it is the change of environment. Here is more information:
https://www.gardeningknowhow.com/houseplants/hpgen/houseplant-dropping-leaves.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm