Simplify the following text for a Grade 11 native English-speaking person.
Original: It is the third installment in the Killzone series that began on the PlayStation 2 with Killzone and continued on the PlayStation Portable with Killzone : Liberation .
Simplified: It is the third game in the Killzone series that began on the PlayStation 2 with Killzone and continued on the PlayStation Portable with Killzone : Liberation .