These both work great, but do that for the whole document? Is there a way to restrict it to only apply the lines to some parts of the file, not every single paragraph?
Thanks!
--Joel

    On Monday, October 10, 2022 at 04:57:07 PM MDT, Max Chernoff <mseven@telus.net> wrote:  
 
 Hi Joel,

On Mon, 2022-10-10 at 12:46 +0000, Joel wrote:
>  Hello Max,
> It is preferred if the solution is just three lines per paragraph,
> rather than some content parallel to the text

A Lua callback solution:

  \startluacode
      -- Constants
      RULE_OFFSET    = tex.sp "1em"
      RULE_THICKNESS = tex.sp "0.4pt"
      RULE_LENGTH    = tex.sp "3cm"
  
      -- Callback
      function userdata.lines(head)
          if status.output_active or
              tex.nest.ptr > 1
          then
              return head
          end
  
          local i = 0
          for n in node.traverseid(node.id "hlist", head) do
              i = i + 1
              if i > 3 then
                  break
              end
  
              local offset = node.new "glue"
              offset.width = RULE_OFFSET
              node.slide(n.list).next = offset
  
              local rule = node.new "rule"
              rule.width = RULE_LENGTH
              rule.height = RULE_THICKNESS
              rule.depth = 0
              offset.next = rule
          end
  
          return head
      end
  
      nodes.tasks.appendaction(
          "finalizers",
          "after",
          "userdata.lines"
      )
  \stopluacode
  
  \parskip=\baselineskip
  
  \starttext
      One line paragraph
  
      Two line paragraph \\
      Two line paragraph
  
      Three line paragraph \\
      Three line paragraph \\
      Three line paragraph
  
      Four line paragraph \\
      Four line paragraph \\
      Four line paragraph \\
      Four line paragraph
  
      \samplefile{bryson}
  
      \samplefile{knuth}
  \stoptext

An \everypar solution:

  \appendtoks%
      \vbox to 0pt{%
          \dorecurse{3}{%
              \rlap{%
                  \hskip\dimexpr\hsize+1em%
                  \vrule height 0.4pt width 3cm%
                  \relax%
              }%
          }%
      }%
  \to\everypar
  
  \parskip=\baselineskip
  
  \starttext
      One line paragraph
  
      Two line paragraph \\
      Two line paragraph
  
      Three line paragraph \\
      Three line paragraph \\
      Three line paragraph
  
      Four line paragraph \\
      Four line paragraph \\
      Four line paragraph \\
      Four line paragraph
  
      \samplefile{bryson}
  
      \samplefile{knuth}
  \stoptext

Neither of these solutions are great though. Both of these solutions are
pretty low-level, so there's presumably a more ConTeXt-y way of doing
this.

Thanks,
-- Max