Points are;
-A C3 master computer can control either up to 3 slaves or up to 3 masters, but not both.
-A C3 network cannot include more than 12 units, regardless of the numbers of computers on it.
-A unit can use as much masters on it, while cannot use more than one slave one it.
Therefore it is possible;
-Two masters can hold three slaves each, one master can hold two slaves, then finally one master can hold three masters mentioned earlier, making total 12 units of company level control with 4 masters and 8 slaves.
-Two masters can hold three slaves each, and one unit with two masters can use one to hold three slaves, while the other master can hold two masters mentioned earlier as well as the another master the same unit has, making total 12 units of company level control with 2 masters, 9 slaves and 1 twin master unit.
So that's my understanding of the diagram in TW. In short, it's not so efficient choice for tonnage wise for you need four masters anyways and even needs for one more slave on the company, but you need one less unit with bears a master therefore one more unit is risk free from sustain the network, and you need to hide less numbers of master units to protect.