These bases are examined in terms of their ability to donate electrons to a hydrogen atom. The carboxylate ion is stabilized by resonance, thus making it a poorer base. The thiolate and alkoxide ions do not enjoy this resonance stabilization, have a stronger driving force to dispose of charge by sharing electrons, and are thus stronger bases than the carboxylate. The oxygen of the alkoxide is smaller and therefore harder than the sulfur of the thiolate. Thus an alkoxide is a harder base than a thiolate.
Protons are small and therefore hard.
Because hard reacts best with hard, the alkoxide will be a stronger base than the thiolate. Using these two factors we have ranked the three bases, so we do not need to consider electronegativity or inductive effects.