Years ago I remember going from some cheapo wide Weinmann rims to some Mavic 317…..same tyres, but on the first ride I really noticed the Mavic being narrower. More skittish/lively. Can’t remember which Weinman rim it was.

So, I now have some rather beaten up and flat spotted Stans Olympic rims and looking to replace them. Would I notice the ~3mm wider Crest? Or would I have to go up to the Flow ~6mm wider before I really noticed a difference?

Strength isn’t an issue, I know the Flows will be much tougher.

more stability in corners on a wider rim, less chance of rolling the tyre off.
wide rims a better suited to heavier riders too.

But in the grand scheme of things is ~3mm naff all, or would I notice it?

i doubt it mate.

Would you notice what?
never ridden the rims you mention (well, except flows but not properly) but I def notice the difference in stiffness between different rims pretty quickly.
for me, I find there is a sweet spot for rim width to give my tyres the profile I like. (a nice square profile with a definitive edge). too narrow and the profile becomes too round, too wide and the edge knob grip changes).

if you are beating up and flat spotting your current rims I’d be looking for both stronger and stiffer.

Not sure about whether or not you’d notice 3mm but when I went from 317s to flows, it was like the difference between night and day.