SICP Exercise 1.3: Sum of squares of two largest numbers out of three
Finding the minimum of three numbers is easier than sorting three numbers, and it doesn't require creating a temporary array.

This runs about 10 times faster.

var sumSquareNotMin = function(x, y, z) {
    var min = Math.min(x, y, z);
    return -min * min + x * x + y * y + z * z;
};


As @Corbin mentioned, floating-point arithmetic is tricky.  I've arranged the negative term first, which should cope better with overflow.  However, in extremely unusual cases (such as when dealing with a mix of tiny and huge numbers), the result could differ from the original.