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Here is an article that you may find helpful: https://www.gardeningknowhow.com/houseplants/kentia-palm/howea-forsteriana-kentia-palm.htm

It looks like the plant certainly has powdery mildew, which is a fungus and can be treated with a fungicide.

It looks like the plant may also have scale, which would cause the black spots. This needs to be treated with a pesticide.

It sounds like your tree has a rust fungus. This article will help:
https://www.gardeningknowhow.com/plant-problems/disease/learn-about-plant-rust-disease-and-rust-treatment.htm

Try pruning out any discolored leaves. Neem oil is also effective treatment for the scale issue. This article will help you with the scale: https://www.gardeningknowhow.com/houseplants/palms/sticky-palm-tree-leaves-treatment-for-palm-scale.htm

If the roots are showing above the soil, it does sound like it may be time for a new pot. I would make the new pot only about 2 inches larger than the current one as Kentia palms grow best when slightly rootbound. This article will help: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm

Plants, like animals, have a life span. At 25 years, it is very possible that they are dying of simple old age.

But then again, as all 3 are dying together, it could also be conditions that are doing so. What those are could be varied, but look to what has changed for these plants lately. Did the heating in your home change? (this would change the temps or humidity in the house) Did the light change? (Perhaps something that blocked the light or caused more light) Do that plants have a root disease? (this article may help with that: https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm

You need to find out what has changed to make the plants unhappy. But, as I said, keep in mind that they may simply be dying of old age, for which there is no fix for.