Not a hundred percent sure as to what you're really trying to accomplish, but the simple answer is no you do not have to quiesce/break the replication between B and C prior to reversing A and B because you're not really affecting the relationship from B to C. In effect, you're turning B into the "source" for both A and C.
For example... this is "B" before the reversal (1netapp03 -> 1bon01 -> 5netapp01)...
san-1bon01> snapmirror status
Snapmirror is on.
Source Destination State Lag Status
SAN-1NETAPP03:test_vol san-1bon01:test_vol Snapmirrored 00:11:31 Idle
san-1bon01:test_vol San-5netapp01:test_vol Source 00:11:31 Idle
After I quiesce and break the A->B relationship, then "reverse resync" B->A (without doing anything to B->C relationship)...
san-1bon01> snapmirror status
Snapmirror is on.
Source Destination State Lag Status
SAN-1NETAPP03:test_vol san-1bon01:test_vol Broken-off 00:21:23 Idle
san-1bon01:test_vol SAN-1NETAPP03:test_vol Source 00:05:20 Idle
san-1bon01:test_vol San-5netapp01:test_vol Source 00:21:23 Idle
san-1bon01>
As you can see, B is now "source" to A and C.
To go back to normal, you update/quiesce/break the B->A relationship then resync the A->B relationship...
san-1bon01> snapmirror status
Snapmirror is on.
Source Destination State Lag Status
SAN-1NETAPP03:test_vol san-1bon01:test_vol Snapmirrored 00:00:14 Idle
san-1bon01:test_vol SAN-1NETAPP03:test_vol Source 00:11:21 Idle
san-1bon01:test_vol San-5netapp01:test_vol Source 00:27:24 Idle
san-1bon01>
You can then delete the B->A relationship that was created when you reversed and after that you should be back to normal...
san-1bon01> snapmirror status
Snapmirror is on.
Source Destination State Lag Status
SAN-1NETAPP03:test_vol san-1bon01:test_vol Snapmirrored 00:03:05 Idle
san-1bon01:test_vol San-5netapp01:test_vol Source 00:30:15 Idle
san-1bon01>
Hope that helps.
