3. Tom and his friend saw how

The text states that Tom and Huck witnessed the murder. It also says, 'The real murderer puts the blame on an innocent man, Muff Potter. Tom and Huck are the only ones who witnessed what really happened. The murderer is a violent man, Injun Joe'. This implies they saw Injun Joe commit the murder. The options are 'Potter stole some things', 'Injun Joe killed a man', and 'Potter hid some treasure'. Based on the text, they saw Injun Joe kill a man.