Answer: I can't tell you for sure what killed them, but Boxwood are susceptible to several soil problems, including a fungal root disease and nematodes. They also hate soggy soil and watering 3 times a week sounds excessive, especially if your soil is a clay.
I would suggest you try dwarf wax myrtle or dwarf yaupon as both are native plants that do well in your area.
Thanks for the question!
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