Handle 3-qubit gates

[tonybruguier]

During the writing of #621 it became apparent that the new version of cirq 0.13.0 had changed the way gates are decomposed. Past quantumlib/Cirq#4167 some gates were decomposed into 3 qubit gates, which are not currently supported. Thus, #621 bypassed Cirq's default decomposition as a work around.

However, the work-around shouldn't say forever. Instead, we should:

• Handle 3-qubit gates
• Revert the work-around

[MichaelBroughton]

Totally agree, one hard part might be dealing with writing down parameter shift rules for 3 qubit gates, but it should still be possible.

[andrelondono]

Whats are coherence times for qubit graphs?