Q.What Is Wrong With My Arborvitae?
I planted these three arborvitaes on the same day. Please see the one that is struggling. What is wrong with it and what can I do to help it? The neighbor is spraying chemicals close to the fence line but I don’t know if that’s the issue or if it’s something else. Why are the two arborvitae’s doing fine and this one is hurting? Also, please see the brown spots on the Yucca plant.

Certified GKH Gardening Expert
It could be herbicide drift if the neighbor is spraying right near that tree. However, arborvitae are susceptible to spider mite and scale damage, as well as wind desiccation and leaf blight.
For the yucca, remove the lower leaves that have the most spots. Then spray the plant with a copper fungicide.
These articles should help:
https://www.gardeningknowhow.com/ornamental/trees/arborvitae/growing-arborvitae-trees.htm
https://www.extension.purdue.edu/extmedia/id/id_184_w.pdf
https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-plant-with-black-spots.htm