See
https://en.m.wikipedia.org/wiki/Barlow_lens
When used on a refractor a 2x Barlow inserted into the star diagonal will produce 2x but if the star diagonal is inserted into the Barlow the extra distance of the star diagonal gives 3x (approx. and dependet on optical length of diagonal). I’ve never tried this or used extension tubes and simply buy and use the appropriate Barlow magnification. I’m not sure if there is any vignetting when adding an extension. Note a 3x Barlow is not double the length of a 2x (e.g. Tele Vue) so there must be a difference in focal length of the Barlow doublet.