LaTeX is a markup language though, not ASCII art. I can get behind two dashes as a substitute if no en dash is available, but three seems too much and looks like halfway to a horizontal line to me ;)

TeX puts more space after periods/fullstops (which is why you're supposed to do special markup or other measures to mark '.' in the middle of sentences which aren't sentence-enders (e.g. like e.g.)). But it's generally smaller than the equivalent of two manual spaces.

(A nice thing in (La)TeX is that one could follow the "two spaces after a full-stop" rule, which then has the advantage of being an explicit marking for sentence boundaries (which your editor might be able to navigate; Emacs has a convention of assuming two spaces after a sentence-ending '.'), but then the TeX typesetting will take care of making it look right. I lost the habit of actually doing this, for better or worse, except when flycheck/checkdoc/package-linter.el makes me do it for docstrings.)

I used to feel similarly. Now I find the double space a visual distraction that doesn't in any way improve readability.

The effect of the double space is, I suspect, a product of the reader's expectations: if you expect it, its absence creates mental work, detracting from readability; if you don't expect it, its presence is what creates mental work.

  Hard habit to break. I learned it so long ago too.

Haha I learned to type organically, and it was only in my mid-40s that I retrained myself to type the correct way. It took something like 40 hours of practice on keybr.com before I could get close enough to my regular typing speed, such that I could switch over to the 'correct' method without it impacting my work.

Retraining myself to stop doing double-spaces took maybe a week.