OF2F has more experience than I do with banggai cardinals, so I will defer to his opinion on this. Keep in mind, however, that just because you have 3 fish does not mean they will pair up. It is just a possibility that you need to be prepared for in the future. If you see this behavior, then you can always return the odd ball to the LFS, or use it in your next aquarium.
I like your coral selection. Keep in mind that regarldless of the size of the colonies you are purchasing, they will spread rather fast. Be sure to allow space, especially for the Xenia and Green Star Polyps. Also, the leather will grow upwards rather quickly, and will shade some light from surrounding corals.
Bottom line, don't stock the tank for what it looks like today. Stock the tank with corals for what it will look like in 6 months.
Supplementation should be calcium and buffer only. Do not add iodine, stontium, molybendium, or any other additives that the industry pushes down your throat. We do not have accurate test kits for these other additives, and you should never add what you can't test for. Instead, you will monitor your water change routine and adjust accordingly based on the appearance of your corals. It sounds funny now, but it becomes obvious over the next few months.