Since you can play the planets in any order, the third robot could be any of the "boss" creatures on the planets.
(see the related questions)
There are robot creatures on Astro Knights island, and you use the Mystical Weapons of Arturus to defeat them on the three planets.
There are no cubes on Astro Knights island. There are ice floes on the Ice Planet, that you jump across to avoid the robot fish.
Nothing. When you defeat the giant robot, you take the Orb back to Arturus and Mordred is left behind in his castle. You receive the island medallion.
There is no jail on Astro Knights Island. There is a castle dungeon, where an attacking alien robot is being held after his capture. (see the related question)
Actually, Merlin has little to do on Astro Knights Island. He recovers the fuel rod from the dungeon when the robot self-destructs. And he battles Mordred at the very end of the island.
you fight the robot on jungle planet
The object of Astro Knights is to get into space using the crashed saucer Excalibur. Once there, you travel to three planets inhabited by robot creatures and defeat them. Then you can travel to the lair of the villain to rescue the Princess. (for a complete solution, see the related question)
One of the robot attackers was captured and put in the castle dungeon.
The solution to Astro Knights Island is at the related question below. Your quest is to rescue the Princess, who has been kidnapped by robot invaders and taken into space. Find and repair the saucer Excalibur and launch into space, to find the three planets where space knights are searching for the princess. Defeat the robot monsters on the planets using the Mystical Weapons of Arturus, then face the Binary Bard in his castle in another galaxy. (see related solution question)
Things that Zap you on Poptropica : Lasers (Spy Island) Electrified Windows (Spy Island) Mini-Bots (Spy Island) Electric Fence (Diamond Mine, Nabooti Island) Electric Caterpillars (Jungle Planet, Astro Knights Island) Mordred's Robot (Astro Knights Island)
When you awaken the robot owl Merlin, he flies out of Mordred's lab. In order to get him back, you have to feed him the robot mouse. (chomp)
Throw the switch when you get the cheese. You can enter the dungeon from Mordred's underground laboratory once you have the robot owl. (see the related question)