I think the point is that that they are Canon's products and, as such, Canon gets to say what they call a "Flagship". For the last several decades, Canon has used the "1" series to denote their Flagship.
It is subjective. There is no standard out there, unless we are suddenly talking about a Navy and which ship the Admiral/ranking officer is on. Although, I think it is worth pointing out, even in the Navy, the "Flagship" is whichever ship the commanding officer is currently aboard...frigate, aircraft carrier or dingy. It isn't always the biggest or best ship.
But the main point of my post is to invert the thought....what if Canon had named the R3 the "R1" in 2021. As some people are questioning if the R1 even is a true "flagship," just take a second and consider the features and more mature technology that is in the R1. A great summary, scroll down to the alternatives in
TDP's R1 review. And flip this around, the list of advantages of the R3 over the R1....
- "4.15 million dot LCD vs. 2.1 million
- modestly smaller and lighter
- Lower price"
...that is not a relevant list if you want to argue about "Flagship". The list of advantages of the R1 over the R3 has 21 bullet points. The R1 appears to be better in many many ways.
Keep in mind, a "3" in Canon's naming structure is still very high level, so this is no disrespect to the R3, a very impressive camera. It is not surprising that there is some confusion as to just how good it is...the same is true with the R5 with some talking about how similar it is to the R1.
As you have seemed interested in a few specifics that Canon, at this time, reserves for the "1" series:
- Dual card slots of the same CFe Type B card
- Best EVF
- Largest buffer/most continuous shooting
- It's not in people's hands yet, but it does seem that there may be some ergonomic/size differences (R1 is same height as 1DXIII)
- I also suspect the AF is the best in the R1, although the R5 shares many of the AF/AE specs.
As to why was the R3 not the original R1, I suspect Canon wanted to test out eye-controlled AF, get the readout speed to be faster than mechanical so that ES was as good as mechanical shutter, and have some sort of vertical AF solution. Also, it was just an awkward transition time with the 1DXIII released in 2020.
In the end, they are simply great cameras...all of them very high end. It is not shocking that the differences are somewhat minor.