This is why I almost always wait for second gen of a new kind of hardware. Eg, my current CPU is a Sandy Bridge. When the i series came out I wanted one badly, but I waited and on the first week of launch bought second gen.
The same with the upcoming Macbooks set to be demoed on the 10th. If I want one, I'll wait for at least second gen.
Regarding AMD CPUs this is an unusual situation, because 3rd gen or 4th gen may be optimal, which is highly uncommon. This is because next gen will run on faster ram. The bottleneck on the Zen 2 processors was cache delays. The bottleneck on the Zen 3 processors is ram delay. This shows to me there is more improvements to come. However, unfortunately, this also means cost will go up quite a bit in newer generations with smaller gains making it not as worth it. I don't game much and I do data science work, so processing is done in the cloud, so I have virtually no reason to upgrade my nearly 10 year old CPU as odd as that may sound.
It looks like AMD actually has two teams working on Zen. So Zen1 was from the first team, Zen2 was refinement of Zen1, and Zen3 is the first gen from the second team. So if you follow your rule, you might want to wait for Zen4, although it isn't clear to what degree the teams work together (if team 2 works pretty closely with team 1, then this could be thought of as a big refinement with and 'outside' point of view looking at it).
It's arbitrary. If you count Core i series from Nehalem, Westmere is 2nd so Sandy Bridge is 3rd. Count for * bridge, Sandy is 1st. Additionally Zen+ is 2nd and it's least improved generation in this time.
My general preference: buy Tock generation (but Intel recently no Ticks for desktop, AMD does Tick+Tock on Zen2)
The same with the upcoming Macbooks set to be demoed on the 10th. If I want one, I'll wait for at least second gen.
Regarding AMD CPUs this is an unusual situation, because 3rd gen or 4th gen may be optimal, which is highly uncommon. This is because next gen will run on faster ram. The bottleneck on the Zen 2 processors was cache delays. The bottleneck on the Zen 3 processors is ram delay. This shows to me there is more improvements to come. However, unfortunately, this also means cost will go up quite a bit in newer generations with smaller gains making it not as worth it. I don't game much and I do data science work, so processing is done in the cloud, so I have virtually no reason to upgrade my nearly 10 year old CPU as odd as that may sound.