The men pleaded guilty on Friday in federal court in Norfolk, Va. They are among 15 people from Somalia and Yemen facing charges related to the February hijacking of the yacht, Quest.
During the hearings, two of the men also pleaded guilty to hostage taking resulting in death. Those men were considered leaders among the crew, and one of them was the commander of the pirates' boat when it left Somalia.
The yacht's owners, Jean and Scott Adam of Marina del Rey and their friends from Seattle were shot to death after being taken hostage.
Each of the three men could face mandatory life sentences, although they struck deals with prosecutors that could allow them to receive lighter sentences and eventually be deported to Somalia.
The Adams and their friends were the first U.S. citizens killed in a wave of pirate attacks that have plagued the Gulf of Aden and the Indian Ocean in recent years. Prosecutors said the men intended to bring the Americans to Somalia and hold them for ransom there.
The Associated Press contributed to this report.