各執鐵杵 Three Able Imps Take Their Posts
小妖叩頭謝恩，叫點妖怪。即將洞中大小妖精點起，果然選出三個有能的小妖，俱變做老妖，各執鐵杵，埋伏等待唐僧不題。小妖叩头谢恩，叫点妖怪。即将洞中大小妖精点起，果然选出三个有能的小妖，俱变做老妖，各执铁杵，埋伏等待唐僧不题。
逐句解 Line by line
The imp knocked his head on the ground in thanks for the favour and called for the muster.
叩頭謝恩 is the full court formula for receiving a favour from a throne — the same words the king of the Dharma-Revering Kingdom used to Pilgrim at the top of the chapter. The cave keeps the forms.
The imps of the cave, big and small, were mustered at once, and sure enough three capable ones were picked out. All three changed into the old fiend, each took an iron pestle, and they lay in ambush waiting for the Tang Monk; of them we say no more.
將 here is the disposal marker jiāng, pushing the object in front of the verb. 果然, sure enough, confirms the plan step by step as it is carried out — the novel narrates the execution in the same order the imp proposed it, which makes the whole thing feel already decided. 不題 drops the thread and the story goes back to the road.
The plan is executed exactly as described, in the words it was described in. That repetition is not laziness; it is the novel showing you a machine running.
Three copies of the commander are now sitting in three places on the road, and the party the reader last saw is riding up it in single file with Bajie in front.