Hi @s5004675,
Canvas is designed to only have one grade per discussion, not any separate or auto grading for posts/replies. The easiest way I know of around this is to create a rubric for the discussion with the points set up as you desire. You can have a criterion for the initial post worth 10 poins, and then 3 criterions for replies worth 5 points (or whatever scheme you desire). When you grade a discussion in speedgrader, you'll see the rubric on the right and the discussion posts made by the student will be available for you to see on the left. Simply look at the discussion posts and put appropriate points in the rubric. Some people say it's a bit kludgy, but it should get the job done.
Hope this helps!
-Chris