yeah carbon suboxide has a linear structure. Its bond notation structure is
O=C=C=C=O
the end oxygen have 2 lone pairs each . Therefore F.C. on O = 6-2(bonds attached)-4(unbonded electrons ) = 0
the 3 middle C have sp hybridisation
because={ 4 (no. of valence electrons in non bonded state) - 0 (no. of univalent atoms attached ...... note O is not a univalent atom)}/2 = 2 therefore sp hybrid.