I saw that trio too but I already have a pair. RC recommends a 10' tank. Personally I think an 96"x36"x24" (360g) will be ok for them provided that there are not many other large fish, and the tank has vigorous flow. A 120"x36"x30" (560g) will be the ideal tank size though.
If you've never had a large tank or kept large fish before, a tank this size may seem absurd but for a fish that will get to 18" adult (perhaps around 14-15" in captivity), this actually makes a lot of sense. If you haven't seen a full-grown naso tang, go to a public aquarium and take a look. They not only get long, but get very wide as well. At full-adult size they probably measure 3" wide. My male is only about 8" now but is already close to 2" wide. These fish weigh a ton and really need big tanks.