Well, I don't know what is wrong but it sounds like the Barlows are reversed.
First, yes, you are exactly right. With a Barlow, as you add magnfication, you have to rack out the focuser, and with a standard Barlow type optic (not a telecentric), the more amplification, the more outward focuser travel or spaces you should require.
The Baader 1.7x GPC, which actually gives closer to 1.5x than 1.7x, will extend the light path by about 31mm to 32mm so it sounds like your "3x" GPC is actually the 1.8x GPC. A 2.6x GPC extends the light path about 88mm or so (I forget, you can look it up in the Baader Maxbright owner's guide).
A true 3x would probably extend the light path to the same as the binoviewer so in other words, it would make the binoviewer about parfocal to a regular eyepiece.
So, it sounds like they are marked wrong.
This should be easy to test. If the 3x is actually working at 1.8, you should be able to measure the actual magnifcation (count pickets in a fence or roof tiles or something and compare at native and 1.8x to see if you only have about half the number visible).
If it is a case of the elements being reversed you can contact the seller and ask them to send you a pair that they have tested, or if possible, you could just change the elements yourself.
I don't know if this is your problem, but it sounds like it could be your problem. A 1.8x should only shorten the light path to something close to what you report the 3x doing, and since the 1.8x is taking far to much out-travel, then it seems to be a real possibility that they are reversed or mis-marked.