Personally, I wouldn't add one. 3 inches isn't too much space for a plate coral. They are a bit hard to find in small version like an inch or so but putting that size in a 3-inch area is a bit too much. 3-inch diameter turns into about 1 inch left around a plate if you get one around 1-2 inches. So doesn't leave much room for it to grow.
Alongside this, if you have snails and hermits, they will most likely be strolling on the sand and get to the plate coral and have no space to go around it and end up going over it which may irritate it.