Answer: It's good that you're aware that bougainvillea doesn't do well with disturbance. But if you are willing to experiment a little, you could have fun with your new purchase. It is possible to separate your 3 plants, but since you are wanting a tree, or "standard" and since they are already planted together, you can try creating a braided standard. First you must remember that these plants are vines, not trees, and you will have to train them within their limitations.
Choose a large pot, and fill it with soil similar to what they are currently growing in. Get a tall stake and push it into the center. Transplant the 3 bougainvilleas into the pot, spacing them as close together as possible around the stake. Then very carefully begin to braid or wrap the vines around the stake. You may have to do some trimming to remove side branches. Depending on how old or how pliable they are, you may be able to braid them without the stake in the center, but you will still need the stake to support the braid. Tie them gently to the stake with a soft tie so as not to bruise or girdle the stems. If the resulting braid is already as tall as you wish, perhaps 3 feet, then you can allow the vining limbs to grow naturally from there. Many will fall into a weeping pattern. With careful pruning you can shape and train the bougainvillea standard into an interesting form. Most standards, especially braided ones, are trained from a young plant as they grow, so don't be surprised if your older plants don't quite conform to the look you'd like.
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