When you want to show how A, B and C relate to each other.

For example if “you would like to show …“

… that the colors red and blue exist as a single color, but if you combine them, you’ll get purple, then you can use a Venn diagram.

… what the outcome of a series of choices can be, you can use a flowchart.

… how the ranking of teams change over the duration of a competition, you can use a bumpchart.