Somethin to keep in mind is that it's a labor intensive harvest and the products aren't really what we'd want to market. You'd wait until the whole lot of them kind of dies down and harvest winter squash, dry beans, and flint corn then till the remainder back into the soil.

It's not bad for substinence farming, but you're not going to get the yields or reasonable market prices with those crops.

I mean, the three sisters works wonderfully as long as you're not trying to harvest mechanically or minmax your yields. It's a great way to produce food for a family or small village with a comparatively small amount of land and effort.

It's common in backyard and community gardens for a reason. To my understanding: (1) corn is a fast growing, relatively high yield calorie and carbohydrate source and provides a natural trellis for (2) beans which stabilize the stalks and provide a protein and vitamin source, while (3) squash serves as a ground cover to retain moisture, limit weed growth, and produces fruit high in fiber and vitamin C.

The beans do fix nitrogen but this only plays a role in replanting for the next season, reducing or eliminating the need to fallow.

Did they test the soil and plant a control crop of corn only in nearby similar conditions?

When it comes to gardening I’ve heard countless stories of people trying some intervention and declaring “it works for me” without ever having tested the conditions or using a control. Those kinds of results can be safely ignored as unscientific.

Heck, one thing I’ve noticed with gardening is that even if you attempt to plant a bunch of crops under identical conditions (as best as you can manage) there’s so much variability that you get widely varying yields from one plant to another for completely unknown reasons.