They way they work is different than normal Cactus. They "finished" cactus will spawn fruits after rainstorms. Tried planting the 3 pieces of the cactus and tried to make it rain through several different means, but didnt get any fruits to spawn. Suppose it needs an actual rainstorm.Never saw those before Agrarian Skies, so sorry, don't have a proper answer...couldn't you plant the 3 you have and farm them?
Ended up cheating in a stack of fruits so I could just plant some new ones. Already wasted enough time flying around looking for them just for a single cosmetic detail