Where v1 and v2 are of type THREE.Vector3:
function distanceVector( v1, v2 )
{
var dx = v1.x - v2.x;
var dy = v1.y - v2.y;
var dz = v1.z - v2.z;
return Math.sqrt( dx * dx + dy * dy + dz * dz );
}
Update:
In the r74 release of three.js the method .distanceTo( v )
can be used.