The 57-times capped Ireland flanker joined Llanelli from Leeds in 1999 and has made a total of 148 appearances for the club.
Easterby commented: "I'm delighted to re-sign with the Scarlets, especially as we are heading towards some exciting times. The club and region have been a major part of my life over eight years."
Scarlets chief executive Stuart Gallacher said: "We are absolutely delighted that our inspirational club captain and leader has committed himself for another three years. Simon is a world-class player and has become a very important component in the team."