  Majorana zero modes (MZMs) are non-Abelian excitations predicted to emerge at
the edges of topological superconductors. One proposal for realizing a
topological superconductor in one dimension involves a chain of spinless
fermions, coupled through $p$-wave superconducting pairing and electron
hopping. This concept is also known as the Kitaev chain. A minimal two-site
Kitaev chain has recently been experimentally realized using quantum dots (QDs)
coupled through a superconductor. In such a minimal chain, MZMs are
quadratically protected against global perturbations of the QD electrochemical
potentials. However, they are not protected from perturbations of the inter-QD
couplings. In this work, we demonstrate that extending the chain to three sites
offers greater protection than the two-site configuration. The enhanced
protection is evidenced by the stability of the zero-energy modes, which is
robust against variations in both the coupling amplitudes and the
electrochemical potential variations in the constituent QDs. While our device
offers all the desired control of the couplings it does not allow for
superconducting phase control. Our experimental observations are in good
agreement with numerical simulated conductances with phase averaging. Our work
pioneers the development of longer Kitaev chains, a milestone towards
topological protection in QD-based chains.
