This year's Tour starts in Duesseldorf, Germany, on Saturday.
The 32-year-old Briton, who was under contract until the end of next year, has been with Sky since they started up in 2010 and the extension is a vote of confidence in a hugely successful team that has also become mired in controversy.
"I’m really happy here and excited that I’ll be with Team Sky for at least the next three years," Froome said.
"It’s been a really successful partnership and I think one of the big reasons for that is the stability of the team. As a rider that is really important as it means you can just get on with your job and focus on winning bike races."
Questions have been asked about Sky's credibility in the wake of an investigation by UK Anti-Doping into allegations of possible wrongdoing.
This year, Sky have come under further scrutiny with British politicians looking into the circumstances surrounding why a jiffy bag containing medication was delivered to 2012 Tour winner Bradley Wiggins at a race a year earlier.
Sky's team principal Dave Brailsford has said there were "process failures" but has denied any wrongdoing.
"For 2018, 19 and 20 the team's got my full faith," Froome told Sky Sports television on Friday.
"I think certainly in my time with Team Sky I have never seen anything untoward. I don't see any reason why that should change."
(Reporting by Alan Baldwin, editing by Ken Ferris)
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