To find the third bot in the Bubbling Under level of Astro Bot, follow these steps:

1. Enter the Main Area: After entering the large water area, you will see a massive bubble dome surrounded by stone columns and three giant spiky balls circling it.

2. Locate the Wooden Platform: Swim up and look for a sunken ship above the ship. On top of a wooden platform floating on the surface, you will find the third bot. This platform is located in the corner of the stage to the right of the massive starfish.

3. Rescue the Bot: Approach the bot, but be aware that a UFO will snatch it. To rescue it, you need to chase down the UFO using your penguin. Once the UFO is distracted or stops, dash up and out of the water directly into the bot.

By following these steps, you should be able to locate and rescue the third bot in the Bubbling Under level of Astro Bot.