When it is night, when you are facing darkness, are there three or four watches? (There appears to be three.) And the Daf speaks to several threes which, seemingly, are connected. (And, at the risk of going too far in terms of threes, this is Daf 3.)
One is unity, agreement, simplicity.
Two is duality, tension and complexity.
Three is Shalom, harmony, a synthesis of the previous two opposites.