  As quantum circuits increase in size, it is critical to establish scalable
multiqubit fidelity metrics. Here we investigate three-qubit randomized
benchmarking (RB) with fixed-frequency transmon qubits coupled to a common bus
with pairwise microwave-activated interactions (cross-resonance). We measure,
for the first time, a three-qubit error per Clifford of 0.106 for all-to-all
gate connectivity and 0.207 for linear gate connectivity. Furthermore, by
introducing mixed dimensionality simultaneous RB --- simultaneous one- and
two-qubit RB --- we show that the three-qubit errors can be predicted from the
one- and two-qubit errors. However, by introducing certain coherent errors to
the gates we can increase the three-qubit error to 0.302, an increase that is
not predicted by a proportionate increase in the one- and two-qubit errors from
simultaneous RB. This demonstrates three-qubit RB as a unique multiqubit
metric.
