How many stereoisomers does 1,2,3-trimethylcyclopropane have? Count enantiomers separately.

Each methyl is either above or below the ring plane. The all-cis isomer is achiral. The isomer with two methyls on one face and one on the other has a mirror plane through the odd carbon, so it is also achiral. Flipping the ring gives no new compounds, so $2$ stereoisomers exist, not $2^3=8$.