Ho-Oh, Luigia, and Deoxys Are Available In The FireRed/LeafGreen Games But Only By Events.
Deoxys Requires The Aurora Ticket Which Gives You Access To Birth Island Where You Can Battle Deoxys At Level 30. The Aurora Ticket Was Distributed on October 9, 2004 - October 23, 2004.
Here's A Link For More Infomation :
http://www.serebii.net/events/auroraticket.shtml
Ho-Oh Requires The Mystic Ticket Which Gives You Access To Navel Rock Where You Can Battle Ho-Oh At Level 70.
Luigia Also Requires The Mystic Ticket Which Also Gives You Access To Navel Rock Where You Can Battle Luigia At Level 70. The Mystic Ticket Was Distributed on September 17, 2005 - October 22, 2005.
Here's A Link For More Information :
http://www.serebii.net/events/mysticticket.shtml
However Ho-Oh and Luigia Can Still Be Obtained By Trade. Ho-Oh From Pokemon Colosseum and Luigia From Pokemon XD.
Mewtwo Is Available In The FireRed/LeafGreen Games Without Any Events Needed.
In Order To Get Mewtwo. You Will Have To Defeat The Elite Four, Have The National Dex, and Fixed The Network Machine on Island 1. From There You Will Have Access To Cerulean Cave Located on The West Side Of Cerulean City By The Waterway. Mewtwo Is Located Deep In Cerulean Cave At Level 70.
Hope This Helps.