There are times when simply assembling the question is the most difficult step.......

I THINK I notice the conventional method is to use the last oscillator (i.e: OSC 3) as the primary tone source(???).

Any thoughts much appreciated...... one guess is that you can modulate a signal (i.e. OSC1's tone) with OSC 3 and also control the magnitude of THAT modulation from another source like LFO, sequencer, etc.

Just curious why it nearly always appears the case to be that the last oscillator is the "boss" while the others just wait for something to do.

 

If a subtractive synth has 3 oscillators that 3rd one often has the ability to act an LFO. If it only has 2 osc, the second osc will get it if its available. Conversely, if cross modulation only has one source that tends to be on osc 1. One ring mod? Often osc 2. 2 ring mods? osc 1 and 2. Osc 1 and 2 got a toy but 3 of that thing would be silly? Put it on osc 3 (maybe a PW or noise). FM? AM? Continuously variable? Some synths all the oscs get it, some 1... maybe 3 modulates 1, maybe 2 modulates 3. Sub osc? Might be tied to osc 1. Might be it's own thing. All oscs having the ability to modulate any other osc? That exists too. Modulate any other destination anywhere in the chain? Sometimes.

There's no rule that the extra lo freq has to be osc 3. Its just convenient so most makers do it that way.

There is no convention in the sense of any standard across the board as to a 'primary tone' osc. Often there isn't a dominant tone, rather the waveform exiting the osc's stage is the result of blending the sources and all sources being necessary to create the final tone.

All of this can vary considerably depending on the synth. Subtractive synthesis is just the method in which the waveform is shaped, not a prescription for any particular mandate of features or workflow (aside from, you know... subtracting frequencies). There are many variations on how much control you are given to do so.