You can trim them back by 1/3 with no problem. If that means there aren't any more leaves, no problem - they'll just be dormant, which is how many people overwinter these plants.
Where are you going to be keeping them over the winter?
In your house next to a sunny window? If so, they could grow new leaves. In this setting, they could keep blooming. In which case I would trim back half the branches, wait until those grow before trimming the others. That way you'll have blooms for much longer.
If it's in you garage, where the temps are cooler, then dormant it is!
The care required in these two scenarios are very different. In the dormant state, the plants would only need water about once a month. I have a difficult time with over watering my hibiscus in the winter, so use a water meter. They're inexpensive and have saved the life of many of my plants.
Here's a good link with info on overwintering tropical hibiscus in colder climes.
Barb

Here is a link that might be useful: Tropical-Hibiscus

Thanks! They are going to be indoors. They have the sunniest spots in front of several windows. All but two still have a fair number of buds.

I will try cutting back the non-bloomers by a third. For the plants with buds, IÂll trim down the branches without.

Sounds great. Enjoy the winter blooms!
Mine are all the greenhouse now. We had threat of freezing temps last night, so in they went.
Barb