<p>The hybridization of quantum states hosted in materials with very different natures is a key resource for quantum technologies. A main example is light–matter interactions, a cornerstone of many quantum computing architectures. In some instances, coherently interfacing more than two quantum systems is crucial, as required, for example, in frequency conversion. Such multipartite quantum blocks have long been envisioned in the field of magnonics^{<CitationRef CitationID="CR1">1</CitationRef>,<CitationRef CitationID="CR2">2</CitationRef>}, but the observation of coherent interactions between more than two systems has remained elusive so far. Here, by combining an antiferromagnetic crystal, a magnetic-field-resilient superconducting circuit and a microwave cavity, we demonstrate strongly hybridized photon-superconducting circuit–magnon states in a microwave cavity. The anharmonicity of the superconducting circuit enables efficient nonlinear interactions between the three modes, which have very different frequencies. As antiferromagnets are naturally suited for coupling to terahertz signals, our work provides a path towards realizing quantum interfaces between microwave and terahertz radiation^{<CitationRef CitationID="CR3">3</CitationRef>,<CitationRef CitationID="CR4">4</CitationRef>}.</p>