It depends somewhat on the breed, temperament and age of the two other pigs and the kune kune.
Why do you have the other two, and what would the kune kune be for?
If raising for meat, it may be better to wait til the first two go, and get 2 or more kune kune.
If for breeding or pets, it may be better again to get two kune kunes and have separate enclosures for the kune kunes and bigger pigs.
If the other two were castrated males, fairly docile and not too much bigger than the kune kune, they may be able to live together once pecking order has been established.
Another consideration is kune kunes tend to need less feed than bigger pigs so you may have difficulty feeding them correctly if all together.