Yer, lol.
There are only ever two, a master and an apprentice. For said apprentice to succeed his master, logic entails that he must kill his Master, and take on another to keep the rule of two in tact.
And actually, rule of two in terms of killing master/apprentice or keeping it in tact was held true in RoTJ.
And I quote from Sidious. [May be inaccurate, memory is a bit fuzzy.]
"Kill your father and take your place by my side."
Why not just say...
"zomg we shud have triumvirate."
In other words, the idea of one master, one apprentice in terms of ideology was around in the 70's.