Green Giant Arborvitae get really large, much too big for this area. Emerald Green would be a better choice. It will tend to become 15-20 feet tall and 3-4/5 feet wide with time. Arborvitae need a moderate amount of water. Blue Junipers of similar size are available that need less if you wanted to look into that. (They are not as soft and green as the Arborvitae). Monitoring and watering when needed is particularly important during the first couple of years of establishment.

Now is a great time to plant trees and three should be fine there. 

Here is our information on planting: https://extension.umd.edu/resource/planting-tree-or-shrub

Watering your new trees is covered here: https://extension.umd.edu/resource/watering-trees-and-shrubs

You will want to mulch the whole area to a depth of no more than 3 inches or so, but don't allow the mulch to lean against the trunks of the trees.

Christine