Missing William Tyrrell's foster family vow to 'never give up' searching for him

The foster family of missing Sydney boy William Tyrrell has vowed to never give up searching for their "precious little boy", on the eve of the third anniversary of his disappearance.

The three-year-old was last seen on September 12, 2014, in the front garden of a home in Kendall, south of Port Macquarie, with NSW police a year ago offering a $1 million reward for information leading to his return.

William's family took to Facebook on Monday to say how the past three years have involved "unspeakable heartbreak and endless tears".

"William, we will never stop loving you," the statement said.