... Speaking of Don Pensack... he mentioned this in another topic thread. With a refractor if you put your 2X Barlow ahead of the diagonal, it will be 3X. This gives you more flexibility for your set of oculars ("eyepieces"). For myself, one advantage is that it allows me to use more than one filter at a time.
Venus is very bright. In order to see the disk and its illumination with minimal glare, I have found it convenient to use two filters. That worked for my National Geographic 70-mm refractor, but did not work for my Explore Scientific 102-mm. I could not draw the focusser tube in close enough. On another board, a moderator recommended that I buy a field flattener, a $250 fix to a $250 telescope. However, I found that with the 2X Barlow ahead of the diagonal, the new focal length is so long that I can draw the focusser in close enough with two filters on the ocular and get a nice view of Venus.
Also, of course, if you like experimenting, you can see for yourself the differences among this times two and that times three.