They should be okay being placed near each other. The biggest problem will be overgrowth ( 3 fast-growing corals within a small area will eventually lead to competition for space). So, you may want to account for this and give them as much room for growth as possible. Although some Xeniids and Zoanthids can be toxic, any allelopathic competition between them is unnoticeable IME. The same goes for offensive or defensive aggression with nematocysts.
Basically, provide room for growth and you will be fine. :thumbsup: