You need one master per lance and then one more master to connect all the lance masters together.
Ie
Company master -> Lance master 1 + Lance master 2 + Lance master 3
Slave 1 Slave 1 Slave 1
Slave 2 Slave 2 Slave 2
(Slave 3)* Slave 3 Slave 3
*Only if Company master and Lance master 1 are the same mech.
One more note: each lance can replace all it's slaves with masters, if it wants to, though those masters could not connect to more units. This is of course inefficient, but if that's the mechs you have available, it's a backup option.
That said, company level C3 runs into the problem that if you're actually playing on enough table space to justify a full company, you might end up with C3 being impractical due to being out of physical weapon range a lot (you're looking at 8*2 or 3*3 mapsheets recommended, some players suggest even more).