True, this confusion is introduced by MS terminology. Even though they say it as 3 synch replicas P, Sec, Sec , in reality it's only 2 Synch pairs (P - S1, P-S2) or 2 sync mirroring. And again they say 2 auto failovers but in fact it's just one pair ( P- S1, NOT P-S2) i.e like one synch mirroring setup.
http://msdn.microsoft.com/en-us/library/ff877931
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SQLBuddy