Simplify the following text for a Grade 6 native English-speaking person.
Original: Doom 3 was a critical and commercial success for id Software ; by the beginning of 2007 , over 3.5 million copies of Doom 3 had been sold , making it the most successful project by id Software to date .
Simplified: Doom 3 ( or Doom III in roman numerals ) , is a sequel to the first-person shooter game Doom 2 . It was created by iD Software , and came out in 2004 .