It's not really a question of fairness.  People go on Patreon and pay every month because they want to support the artist.  They don't expect that everyone who gets the game will be patrons.  After all, if somebody is willing to spend $3 for each update, most of them would also be okay with spending $5 a month to be a patron, so there's almost no point in being on Itch at all at that point.

People who buy games aren't the same people as the ones who patronize artists.  People who buy a game want to pay once, and be done with it.  Trying to force a Patreon model on Itch is likely to result in making less money on both sites.

Once a story is far enough along that it seems likely that the story won't suddenly stop next week, it's pretty easy to justify spending $10 or whatever as a one-time purchase.  It's far harder to justify spending $3 per version, having no idea whether there will be one more version or twenty.  So most people's default reaction when they see this happening is to wait for the final version.  By doing so, they're not providing ongoing income during development, which is pretty much the opposite of what you're trying to achieve.  So this approach is likely to be self-defeating.

Similarly, by providing frequent versions for $3 per version, people who might otherwise have been paying patrons to get the latest version are more likely to just spend $3 per version on Itch, so you're losing potential Patreon income as well.

That makes this approximately the worst of both worlds.

Some better ways to differentiate between the Patreon experience and the normal purchase experience include making updates more frequently on Patreon, providing Patreon-only bonus content, giving Patreon subscribers the opportunity to shape the direction of the game, or some combination of the above.  And that seems to work pretty well.

But the best model I've seen so far is this:

• Each version is released a month (or even two or three months) after it becomes available to Patreon subscribers.
  
• That version always costs money, but comes with free updates up to and including the final version.
• When each new version is released, the previous release becomes free.
  
• The purchase price for getting the latest released version goes up as you get closer to the final release.
• The final version won't ever become free, because there's no next version.

The increasing pricing encourages people to pay for it earlier rather than waiting until the final version.   The promise that the final version will not be free eliminates the most likely reason why someone might choose to not pay for it at all.  The availability of a fairly recent free version lets you know almost exactly what you're getting for your money, at least up to that point.  And if you announce price increases far enough ahead of time, they can be a strong motivator, causing a boost in sales.

Just my thoughts.