The word discussion is just like any other noun:
This means you are having more than one discussion with Jack over a period of time.1, I am involved in discussions with Jack...
a. We had a good discussion. = We had one discussion, and it was good.2, We had a good discussion, or We have good discussion, or we have good discussions.
We have good discussion.This is ungrammatical.
c. We have good discussions. = We have good discussions on a regular basis (simple present = habitual action), thus more than one discussion in total.
Student or Learner