Answer: Your plan sounds lovely! It is possible to purchase large trees, but their installation usually requires heavy equipment, and can become expensive as a result. A local garden designer, landscape architect, or larger garden center can give you an estimate on such an installation. Just a word to the wise: larger trees tend to take longer to become established, and in many cases a smaller tree will catch up to the larger within just a few years.
In terms of selecting the tree, the three you mention are all relatively fast growing and tend to establish fairly easily. For quick shade, however, an arbor covered with a vine would give you more immediate results, with the added bonus of not providing so much root competition for your other plants, or rooting into the pond, or overly shading the pond. Good luck!
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