Answer: Hibiscus and princess flower (Tibouchina) are both tropical shrubs (not annuals), so I'd definately bring them indoors. If they are not too large, I'd wait to prune until late winter/early spring just before new growth begins. Otherwise, cut back about 1/3 of the plant right after the bloom cycle. Place in a bright location, water when top inch of soil dries out, and fertilize once a month with a balanced fertilizer.
I'm not exactly sure what plant you mean by potato bush, as it is a common name. Potato vine (Solanum jasminoides) can be cut back severely at just about any time, as it vigorously regrows.
If possible, slowly adjust the plants to bringing them indoors, so it's not as much of a shock. Move them to increasingly more sheltered locations for longer periods. Unless frost is forecast, in which case protect them quickly! Good luck!
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