|I recently planted 3 Thuja occidentalis
|Unfortunately based on your description I am unable to tell you why they are beginning to brown. This is not a good sign, so I would strongly suggest you work with your retailer (especially if there is a warranty) and/or your county extension to determine what is happening. It might be transplant shock (midsummer is the most difficult season for planting) or over or underwatering or some other problem. In the meantime, the soil should be kept evenly moist like a wrung out sponge, not sopping wet and not bone dry. Using several inches of organic mulch in a flat layer over the root area will help with that. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. It is better to water slowly and deeply less often than to do a light sprinkling every day. After watering, wait a few hours and dig down to see how far the water went, it can be surprising. Good luck with your arborvitaes.