I think there's a lot of truth to the notion that it really depends on the kids - which is funny, because that wasn't what I was thinking about when I posed the question. I was looking more at the family dynamic (for instance, is the 3rd hardest because there are still two younger ones who aren't all that independent, but from the 4th onward they older ones are more able to help?), but, clearly, even there there are no cut and dry answers. But we knew that, didn't we?
My 3rd we thought would be the easiest, and in some ways, he has been. We were more relaxed about the pregnancy and his arrival than with the others, much because of experience, etc. We'd not only already had a baby, we'd already had a home birth/VBAC, and so there wasn't much new. He started out as such an easy, happy baby! I said it was wonderful that the 3rd would be the easy one, since I surely needed it by then! I was pretty wasted by the pregnancy - long-time health issues flared badly, basically, so I was mostly worthless for anything that needed to be done - and getting over that with a sweet baby was a pretty good thing!
Then he started moving.
He was rolling at 3 months, crawling at 5 months, standing at 9, and walking at 10. He is NON-STOP, even now at 2.25yo.
Super curious, active, happy, intense, and INTO EVERYTHING. He's the first one we ever bought baby gates for!!
So, it's been a whole different experience from the first two, and definitely a challenge for all of us. Mostly, it's just the stress of having so much going on - noise, chaos, destruction, play, singing, dancing, etc., etc. It's not all bad, but it CAN be all a bit maddening when you're already maxed sensorily. (And I think both DH and I tend to be overstimulated by sensory input.)
We've looked seriously at a new house where the boys will have more freedom to do all their boy things - like make lots of noise - in an environment where it won't be so hard on DH and me. But, for right now, we're trying to stay put and just fix things as best we can here. And, I think if we make the changes we're planning on (DH working at an out-of-home office most of the time), that it will be easier on him to deal with the kids the rest of the time. And, then, maybe we'll be okay to handle a fourth. Maybe?