Assuming the focal length of the barlow is ~97mm (measured from centre of the objective to the end of the barrel) you'd need to add another 97mm to achieve 3x magnification.
ie. doubling the focal length of a 2x barlow will give 3x magnification.
http://www.astunit.com/tutorials/barlow.htm
You could get approximately the needed separation by screwing the barlow element into the nose of a 2" diagonal and inserting the barlow barrel into the eyepiece end of the diagonal. If you wanted more separation you could draw the barlow barrel out of the diagonal a little before tightening the set screw.[edit: I see you have a dob so scratch the diagonal idea]