In which species are all the carbon atoms considered to be ${\mathrm{sp}}^2$ hybridized?

Due to the participation of all s and p orbitals in bond formation, alkanes have ${\mathrm{sp}}^3$ hybridized carbon. Alkenes have ${\mathrm{sp}}^2$ hybridized carbon with a double bond because bond formation only involves one s and two p orbitals. A p-orbital is empty in one.
As only one s and one p orbital participate in the creation of the bond, alkynes have sp hybridized carbon with triple bonds. Two p-orbital is vacant at this time.