The 28-year-old made it clear, though, that he had not given up on a return to play for New Zealand in 2015 when they attempt to retain the World Cup he helped them win 2011.
"It's been a tough couple of days but I am happy to have my future sorted," Williams told reporters at a training camp for the New Zealand rugby league World Cup squad in Sydney on Sunday.
"I will go back to the New Zealand Rugby Union to try and put my foot in the door for a franchise in New Zealand, hopefully the Chiefs.
"But I understand, with the situation of staying in league, that might not be an option.
"But it's good to have made my intentions clear for the next three years. I wanted to put it all out there as I am overhearing what I am going to do as everyone else."
Controversy is never far from Williams and even his decision to play in the upcoming league World Cup involved a backflip on his original plan and another player being dropped.
With Williams also hoping to feature in the New Zealand sevens team at the 2016 Olympics, he said he had decided to put his boxing career on hold.
"I won't have the time to box in the next three years," he said.
The NZRU had better luck in their negotiations with All Blacks number eight Kieran Read and lock Sam Whitelock, who on Sunday announced they had committed to playing on with the national team and Canterbury Crusaders until 2017.
"The fact that two world-class players in Kieran and Sam have re-signed for four more years is a huge positive for the All Blacks," All Blacks coach Steve Hansen said in a news release.
"Not only are they part of the current success of the team, but it means we have a nucleus of experienced players to take the team into the future beyond 2015, which is fantastic." (Reportying by Nick Mulvenney, editing by Greg Stutchbury)
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