I disagree, when looking at 3-kids setup out there in the world. Ie my wife comes from such, she is the oldest and all 3 follow the typical 3 kids pattern - oldest was pushed into most responsibility to the point of trauma, middle one feels unappreciated and has inferiority complex as middle ones often do, and youngest is by huge margin the most spoiled one. This is the default for 3 kids that many parents end up with, unless fighting hard against this natural direction.

And yes a lot of interaction, but it doesn't guarantee anything further down the line - when long time adults youngest is still spoiled and selfish and not really a pleasant person to be around with for anybody, including siblings and their own mom. Middle one is scared to have kids, a decision that she may end up regretting later easily. And my wife is also carrying her part of the burden, won't bother you with details.

The grass is always greener behind the fence and all that.

There's very few humans that have enough time, energy, skills, wherewithal, and money to devote to 3+ kids to make these sorts of positive "three's company" interactions happen in the modern world. I am not saying that it's impossible, just that it's difficult and doesn't happen naturally like you're implying.

On a personal note, I never had much positive interactions with either of my siblings, certainly not as children as not now that we're in our 40s either. Lots of fighting, absolutely zero closeness or positive feelings.