  Quantum networks are the center of many of the recent advances in quantum
science, not only leading to the discovery of new properties in the foundations
of quantum theory but also allowing for novel communication and cryptography
protocols. It is known that networks beyond that in the paradigmatic Bell's
theorem imply new and sometimes stronger forms of nonclassicality. Due to a
number of practical difficulties, however, the experimental implementation of
such networks remains far less explored. Going beyond what has been previously
tested, here we verify the nonlocality of an experimental triangle network,
consisting of three independent sources of bipartite entangled photon states
interconnecting three distant parties. By performing separable measurements
only and evaluating parallel chained Bell inequalities, we show that such
networks can lead to a genuine form of tripartite nonlocality, where classical
models are unable to mimic the quantum predictions even if some of the parties
are allowed to communicate.
