Probably not a good idea! Also if the other two are already established and one is a female, adding in a larger black storm would actually be the worst move. It will throw off the heirarchy. If anything, the black storm should either be the mid sized or smaller fish. Otherwise you could add more than 1 more and start a harem.
If you do decide to add only one third fish, I would ABSOLUTELY do the following:
Buy an introduction box that can hang or sit in your tank and use it for the new clown. Use it for at least a few days.
Tape one or two small mirrors to the sides of your tank, so the fish can see their reflection.
Make the actual introduction into the tank at night time after white are off, approx 30 mins before blues are off.
Room lights off within 30 minutes on the evening you introduce the fish.
I know this may seem tedious for just one introduction, but you are really creating a tough scenario for yourself!