The three men, who officials said may be homeless, were camped out on a sandbar in the Los Angeles River basin.
Firefighters said the men woke up to the rainwater rising around them. Rescuers said if the men were out there longer, they could have been washed away.
"When we first got here, we could see a little bit of the island and it was just up to their ankles," said Long Beach City Fire Capt. Richard Brandt. "After that, it was already up to their knees. When we made the rescue it was between their knees and their waist."
Despite being wet and cold, the men were in good condition.
Officials said the area is popular for homeless people to camp out and they're trying to get the word out that there is more rain on the way and that people should move to higher ground.