The compound shown is bicyclo[4.3.0]nonane (hydrindane), in which a cyclohexane ring and a cyclopentane ring share the C-C bond between the two bridgehead carbons. Considering configuration only, the number of stereoisomers is

The two bridgehead carbons are stereocentres. The two bridgehead hydrogens can be on the same side (cis-fused) or opposite sides (trans-fused) of the ring system. In the cis isomer a mirror plane passes through the midpoint of the shared bond and perpendicular to it, exchanging the two bridgeheads, so cis-hydrindane is achiral. In the trans isomer that plane is lost and the compound is chiral, existing as a pair of enantiomers. Total = 1 + 2 = 3. (Decalin is different: both its cis and trans isomers are achiral by configuration, giving 2.)