Q.Mums
My white mums were put into the ground 3 weeks ago. They looked great until the past few days. I may have missed a day or two of watering, but have been for past 3 days. To no avail. They are still sagging and sad. What shall I do?

Yes, it does look like the roots have dried and the plant is suffering from lack of moisture.
You can trim the flowers and the plant back to the next buds.
Continue daily water on your plant. Pull the mulch back to check to make sure the soil is moist but not soggy.
It may bounce back.
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/growing-chrysanthemum-flowers.htm
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/wintering-mums.htm