Rome, January 11 - Three-year Italian bonds reached their lowest level since March 2010 on Friday when a successful Treasury auction gave further evidence that the country's borrowing costs are easing. The Treasury sold 3.5 billion BTP bonds that are set to mature in December 2015 at an average interest rate of 1.85%, compared to 2.50% at the last equivalent auction in December. The Treasury also auctioned off 1.5 billion euros' worth of bonds set to mature in 2017. The average interest on the bonds maturing in June 2017 was 2.17%, while it was 2.34% for those maturing in October that year. The spread between 10-year Italian bonds and German Bunds fell 20 basis points to 259 on Thursday, its lowest since July 2011. The spread is a key measure of Italy's borrowing costs and of investor confidence in the country's ability to weather the eurozone crisis. It went over 500 points with yields of around 7%, which are considered unsustainable in the long term, at the peak of the eurozone crisis in 2011.