I have heard it said that Satan, the beast and the false prophet are in the Lake of Fire to suffer eternally. What about unbelievers?
The Lake of Fire is the final abode for all ungodly, both Satan, his angels (i.e., demons), and all unbelieving human beings. The first occupants of the Lake of Fire are the Antichrist and the false prophet, according to Revelation 19.
Rev. 19:20 And the beast was seized, and with him the false prophet who performed the signs in his presence, by which he deceived those who had received the mark of the beast and those who worshiped his image; these two were thrown alive into the lake of fire which burns with brimstone.
After the 1,000-year Kingdom concludes, the Lord judges all unbelieving humanity before throwing all into the Lake of Fire:
Rev. 20:11 Then I saw a great white throne and Him who sat upon it, from whose presence earth and heaven fled away, and no place was found for them.
Rev. 20:12 And I saw the dead, the great and the small, standing before the throne, and books were opened; and another book was opened, which is the book of life; and the dead were judged from the things which were written in the books, according to their deeds.
Rev. 20:13 And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds.
Rev. 20:14 Then death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.
Rev. 20:15 And if anyone’s name was not found written in the book of life, he was thrown into the lake of fire.
All who exist in the Lake of Fire (i.e., Satan, the demons and human beings) remain there conscience and in suffering for an eternity. This is the testimony of Scripture: