For 1,2,3-trimethylcyclopropane (configuration only), which statements are correct?

On a planar three-membered ring the methyls can be all on one face (all-cis) or one methyl on the opposite face to the other two. By the ring symmetry all arrangements reduce to these two. Both have a mirror plane through one ring carbon and the midpoint of the opposite bond, so both are achiral. Hence 2 stereoisomers, none chiral, and the $2^3=8$ prediction fails.