Q.Caring for Bougainvillea plants in pots
I have 3 Bougainvilleas in pots which I have to bring in doors in the winter and have questions regarding their care:
When is the best time to prune, before they come indoors at the end of October, or as we prepare to take them outside at the end of May?
When inside, how often should they be watered and how much water?
When preparing them to move back outside what is the best fertilizer to ensure they flower and when?
A photograph of two of the pots is attached.

As long as they are indoors, the timing of pruning is as important. The best thing would be to cut them back heavily after the blooms have dropped. They will not be picky about ferilizer, and an all-purpose will do just fine. As for watering... You will water them much less. Do not feed through the winter. Only water when the top of the soil dries completely.
Here is an article for more information: https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-winter-care.htm