  An interferometric device is proposed in order to analyze the quartet mode in
biased three-terminal Josephson junctions (TTJs), and to provide experimental
evidence for emergence of a single stationary phase, the so-called quartet
phase. In such a quartet-Superconducting Quantum Interference Device
(quartet-SQUID), the flux sensitivity exhibits period ${hc}/{4e}$, which is the
fingerprint of a transient intermediate state involving two entangled Cooper
pairs. The quartet-SQUID provides two informations: an amplitude that measures
a total ``quartet critical current'', and a phase lapse coming from the
superposition of the following two current components: the quartet supercurrent
that is odd in the quartet phase, and the phase-sensitive multiple Andreev
reflection (phase-MAR) quasiparticle current, that is even in the quartet
phase. This makes a TTJ a generically "$\theta$-junction". Evidence for
phase-MARs plays against conservative scenarii involving synchronization of AC
Josephson currents, based on ``adiabatic'' phase dynamics and RSJ-like models.
