how can I get musescore 3 to leave the spacing alone?

MuseScore 3 has made tremendous improvements int he default placements of things, but this does unfortunately mean that manual adjustments performed to work around the deficiencies in MuseScore 2 will often not make sense any more. So, in some of these case, it may be best simply keep using MuseScore 2 for those particular scores.

However, your particular score is actually a good example of where it would actually be quite simply to get much better results quickly in MuseScore 3. It looks like you entered all your LH fingerings using piano Fingerings rather than LH Guitar Fingerings, which is why you had to laboriously reposition then all manually. It would have been better to have entered them as guitar LH fingerings in the first place. Then you would have need only a small of manual adjustment in 2.3.2, but even so, even less in 3.0.

So, what I recommend is the following:

1) open your score in MuseScore 3, answering "yes" to the reset question (thus, your manual adjustments are removed)
2) right-click one of your LH fingerings (which are displayed above the staff since you entered these as piano fingerings rather than guitar), Select / More / Same Subtype
3) Use the Inspector to change the style to LH Guitar Fingering

This now gives you what you would have by default in MuseScore 3, and I think you'll see, virtually no manual adjustments are needed at all. You can certainly try reapplying some of the particular tweaks you made, but really, there aren't any necessary any more that I can see (in start contrast to 2.3.2, where the defaults created all sorts of collisions and required extensive manual adjustment just to get to a minimal level of readability).

If you did a bunch of manual placement in version 2, you will have to repeat the same work to make it look exactly the same in version 3. It's best to just open the score in version 2, unless you want to repeat the work. Version 2 & 3 can coexist on your computer - and do by default.