Understanding sphericity

Which of the following conditions accurately describes the sphericity assumption?

50 XP

The variances of the differences between all possible pairs of groups (i.e. levels of the independent variable) are different.
The differences of the variances between all possible pairs of groups (i.e. levels of the independent variable) are equal.
The variances of the differences between all possible pairs of groups (i.e. levels of the independent variable) are equal.