Cherrylaurel shrubs, while fairly tolerant of a wide range of conditions, do not do well in soils that are poorly drained or otherwise staying too wet. Is the planting being irrigated often, or does another factor (a downspout outlet, perhaps) keep this site damp?

When they were installed, were the root balls loosened of any tangled or matted growth? If not, this drastic difference in soil type between the native soil and that within the root ball interior can lead to desiccation or flooding of the roots, depending on watering/rainfall and soil type. Make sure mulch isn't touching the crown (where the stem(s) emerge from the soil). If the shrub was planted too deeply, with its roots too buried, that reduces oxygen availability to the fine roots and can promote dieback and rot due to opportunistic fungi.

The off-color portion(s) of the center shrub will not recover and should be pruned out. The cherrylaurel should not need fertilizer, and accidental overuse of a formulation like 10-10-10 risks injuring roots through "burn" from the excessive nutrients, so we suggest not applying more, including on any future replacements. If established plantings begin to show nutrient deficiencies in the future, then perhaps the supplemental nourishment may be useful at that time. If you think the plant is too deep, you can dig it up and replant at the correct depth to encourage root regrowth. The root flare, the point where roots begin to emerge from the main stem, should sit just at or barely below the soil surface. Buried stems can also be enticing to a wood-boring pest, as they preferentially target stressed plants and those whose bark is kept moist from overuse of mulch or excess soil. A borer may not be a factor here since the plant is so young, but we can't rule it out. You can inspect branches you trim off for signs of stained inner wood or tunneling.

There is no fungicide or other direct intervention which will be useful. Instead, all you can do is to trim out the dying section, check on planting depth and mulch proximity, adjust watering as needed so the soil dries a few inches down before irrigation, and monitor it for any other changes. Symptoms manifesting from root loss can be delayed in evergreens, so if the plant continues to decline going forward, too much damage had probably already occurred and the plant will need to be replaced. While there may not be any outward indication of this, soil drainage can vary considerably from one location to another, even within plantings at this spacing. If cherrylaurel continues to fail in this one spot, it makes sense to try a different species in its place that should be better adapted to the core issue, be that rapid drying or retaining water.

Miri