To lose such nice conifers is disappointing. I’m sorry.

Did you dig up the one you lost, to examine the roots? That could tell you a few things. Options include planting depth being too high or too deep. Root rots. Sometimes trees are planted with the twine, burlap, or metal root holders, and as the tree grows it is “strangled”. Circling roots can also girdle and kill a tree years after it was planted.

Describe how you planted the trees. How do you care for them. Irrigation? Fertilization? How do you care for your lawn? There are some lawn care products that can’t be used around conifers.

Do you see any signs of bark damage from insects or cankers?

And last but not least, what type of trees are these? We can determine if they naturally decline in our climate. (Yours looks a bit like one of the colored cedars, like White-tipped Deodar Cedar. The Oregon State Landscapes page shows other common cedar on the right of this page.)

How have the trees been doing since May? Heatwaves can accelerate the visible decline in conifers.

The Extension Service advice is against fertilizer, as it can increase water requirements. This article, “Western Oregon conifers continue to show damage due to drought” gives that and other advice. Usually the needles naturally under the tree provide helpful mulch to hold soil moisture, but other organic mulch could be used.

The only tree I see that is going to live and look good in a landscape is the one in your middle photo. Watch them this winter, but I don’t expect the bare branches will get new needles.

When you remove a tree, check the roots to see if there were problems that led to the plant’s decline. Also, you’ll know what avoid when planting replacements. This publication from WSU shows some common problems and gives instructions for correct planting. “Planting Trees and Shrubs in the Landscape (Home Garden Series)”