Well dry soil is a killer but so is soggy wet soil. Either condition for a new plant in the summer can do damage quickly so even if you fix the problem the damage may already be too extensive. Since you watered regularly perhaps the problem is due to soggy conditions. Other than that the only other cause of such death might be damage from a weed killer used nearby that got on the plants or into their root zone.
What is still in your power to do is to keep the soil evenly moist but not soggy wet. Hopefully those still alive will now get better established and be fine. Fall is a good time to plant with low stress to plants. So if you are going to replace dead plants now is a much better time than summer to do so.
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