[quote author=ldparker link
In general, the sub-plots are all relationship focused. Because the story is more character driven, the sub-plots are to help advance the protagonist's personal development in the series.
From what you say, the sub-plot/s, is/are, an important aspect as to how the protag will change with their character arc over the series and how they will develop through their expriences. You can't have too many or too few scenes that help this on its way as long as you don't lose sight of the overall plot to keep it within whatever is your target genre and readership. Hopefully, how your protag changes will lead to whatever changes in character traits and experiences you have set up as sub-plots which will assist the protag in the final outcorme of the trilogy.
The only thing to avoid is having scenes that will not have a bearing on the outcome that will bore the reader to death and lose their interest if the sub-plot goes off at a tangent. They will spot if you are using the scenes to fill out the story.