ArborVitae turning brown
This discoloration suggests an issue with root health, probably caused by either overly-wet or overly-dry conditions. Given the yellowish "off" color of the entire plant instead of a drier browning starting from the branch tips, overly-wet is more likely. Evergreens can take some time (weeks, months) to display symptoms from damage that began awhile ago.
Even if this trio have been watered the same, slight differences in drainage in this one spot may be causing too much moisture to remain around the roots in wet weather or after irrigation, depriving them of some oxygen. Ailing roots cause leaf discoloration because they can no longer function well enough to supply the foliage with sufficient moisture and nutrition. There is no treatment for this except to try to correct the problem (in the case of over-watering, avoiding irrigation or investigating problematic drainage patterns) and hope the plant has enough vigor left to re-grow healthier roots. You can also inspect the lower trunk for signs of damage, in case wildlife damaged the bark and interrupted the sap flow to the branches, though this would similarly be untreatable.
The browning interior foliage is somewhat normal (in that older leaves on the inner, shaded stems like this do eventually shed on healthy plants), but probably exacerbated in this case into premature and more excessive shedding resulting from the root stress. The fact that this center plant is showing this browning while the others are not further suggests that there is a health issue with this individual.
The plants appear to have a browse line from deer, where they have eaten the lower foliage up to the level they can reach (or prefer to lift their heads). While this doesn't impact the dieback issue, it is something to be aware of in that browsed plants will not regrow this lost foliage even if the deer are kept away in the future.
All you can do for now is to give the plant time to recover (if it can), and monitor the soil moisture prior to any watering by feeling the soil about five or so inches deep near the roots. If damp, the plants are fine; if fairly dry, a good soaking will help when rainfall is sporadic or light. We have had drought in autumn before (just last year as it happens), so periodic moisture checks up until we experience regular frosts is best. No application of fungicides or fertilizer will be of use. If the discoloration worsens, the roots were already beyond saving and the plant will need removal/replacing.
Thank you so much!! Your information was very helpful. We haven't watered this area for a few years since the trees have grown. It must be from all the rain we've had. Thanks again!