Final answer:
In Chapter 3 of "Johnny Tremain", Mrs. Lapham and Dorcas immersed Johnny's hand in cold water to ease the pain.
Step-by-step explanation:
In Chapter 3 of "Johnny Tremain", after Johnny burned his hand, Mrs. Lapham and Dorcas **immersed his hand in cold water to ease the pain**. This action is mentioned in the text when Johnny describes the burning sensation in his hand and how he tried to hold the matches away from his body to escape the fumes. Mrs. Lapham and Dorcas reacted by immediately attending to Johnny's burned hand, showing their concern and care for his well-being.