The Hybridization of carbon in carbon sub oxide  is

Each carbon atom in carbon suboxide (C3​O2​) is sp - hybridized. If carbon is bonded with a double bond from both sides, then the carbon is said to have undergone sp hybridization.

The structure is O=C=C=C=O.
                                    sp    sp     sp

all bonds are single or sigma, the hybridization is sp3

all bonds are single and one pie or positive, the hybridization is sp²

all bonds are single and two pie , the hybridization is sp