That’s a good question, and I absolutely don’t mind you asking. Honestly, I didn’t even make the connection between the “threenager” phenomenon and the fact that all three of ours happened when they were new siblings until I was writing up that earlier post, so take my half-decade old memories here with that particular grain of salt; the real reason may well be “oh we never really thought about the connection.”
When we decided to go for our second, I think it was the fact that our first was a really easy one-year-old at the time, so we were feeling the hubris of new parents with an easy kid. He had been a slightly tricky newborn (medically), but then he turned into the sweetest kid, so we figured we were over the hump; he didn’t become difficult behaviorally until right around the time our second child was born. We also knew we wanted to have more than one, and had seen some research (and had some personal experience) that suggested a two-year age gap was best for sibling bonding and closeness, though I think I’ve heard that data has been questioned more recently.
And then our second was born, and life was kind of hell for a little while because she wasn’t sleeping and he was a threenager (and so you’ll note that the gap between the second and third is definitely bigger than between the other two gaps), but just like it got bad at the same moment, they both straightened up at more or less the same moment, too; he was potty trained, she started sleeping better, and they were suddenly really good friends, so we knew (or thought we knew) that the hard times would end and that we could make it through more or less unscathed.
At that point, we switched from the hubris of new parents to the hubris of parents of a big family, so after the third’s infancy was more or less fine and we made it through the second’s threenager phase, we went for a fourth, assuming we knew the drill. We didn’t; our third child’s absolutely horrible year from 2½-3½ was like nothing we’d experienced before or since. But it also ended, eventually. We had already decided even before the last one was born that we were done at four anyway, so we knew that it could only last for so much longer.
So I guess the tl;dr answer would be, ignorance at first, and after that confidence (hubris?) that we could handle it. In hindsight now, I don’t regret it. I honestly don’t even know if I’d warn new parents not to do it the way we did it if they wanted to, though I would give them the heads up that it’ll be tough for a while, and strongly recommend that they get a good community around them.
I don’t know you or your family at all, of course, so I wouldn’t counsel you either way; but I will say that the people who are unsure about whether they’re up to the task are, in my experience, the people who are most up to it. The people who are confident about it all are probably going to crash and burn, and the people who are sure they aren’t up to it are probably right. But if that worry about the first year overlapping the third is the only thing holding you back, maybe the assurance that it ends eventually will help.
(Sorry about the novel! Hope that helps.)