Hello Max,
It is preferred if the solution is just three lines per paragraph, rather than some content parallel to the text...I'll be modifying whatever solution to fix a bunch of other situations (e.g. display a box next to each paragraph).

--Joel

    On Monday, October 10, 2022 at 12:24:03 AM MDT, Max Chernoff <mseven@telus.net> wrote:  
 
 Hi Joel,

> I'd like to add some area for readers to write in the margins of some
> text. This would leave three lines, like this to the right of the
> text.

Is it okay if there are rules continuously down the right column? If so,
this is fairly simple to do with layers/backgrounds + MetaFun. 

If you want for there to be exactly three rules per paragraph, then that
is more complicated. I can think of some bad solutions (\everypar, Lua
callbacks), but I'll leave it to someone else to think of a good
solution.

Thanks,
-- Max