In reflecting on these three terms, selichah, mechilah and kapparah, they are often thought of simply as synonyms of forgiveness.
But trying to focus on their specific meanings in the context of the judgment of the Heavenly Court and how they are used in the liturgy of Rosh Hashanah and Yom Kippur suggests that they are actually three different and distinct requests which apply to different circumstances.
In particular, when considering the conclusion of the confession of Yom Kippur which speaks of the סלחן לישראל and the מחלן לשבטי ישרון, the first two categories. But then goes further requesting no suffering, afflictions or severe illnesses, which would suggest a connection to kapparah.
How are we to understand the meaning of what we are asking from G-d, the Creator of us all?