I agree. I think the main reason they stated that R3 is between R5 and 1DX III is to let photographers know that they have more in store, that the R camera lineup is still not complete and that they have the technology that will push the mirorless replacement for the 1DX III above R3. Be it with more fps, global shutter, better IQ, more MP, AF, something else or with all of the aboveThat doesn’t really make sense. The fps is not limited to the card write speed but the buffer capabilities, put in a bigger buffer and it will, well, buffer.
If Canon are capable of 40fps, or whatever higher number than 30, they might well be saving that for the R1 if there is one. Canon are saying the R3 is between the R5 and the 1DX III, that implies they will make an ‘R1’. That R1 will have higher fps than the R3.
Besides if they stick with the larger form of the pro bodies, then they have more room for better batterries, heat sinks, processors.... Everything that helps with faster processing.