A 3-year-old definitely takes a lot more continuous attention than a newborn (assuming the newborn is healthy, eats and sleeps okay, ...).
I was able to get some amount of work done while taking care of my first kid up to about age 1. In the two years since then, not so much (I just pray for conveniently timed naps). The second one is 6 months old, and is way easier to take care of right now.
If you just accept that you will be doing no independent creative work, then a 2+ year old isn’t too bad. They can start to “help” with routine chores, can walk around for themselves, can feed themselves, eventually stop needing diapers, etc. The work is not really hard per se. More like relentless.
Sometimes friends or family members ask me why XYZ projects are half completed, and it's hard to properly find the wording for "I want my wife to still like me at the end of most days".
This is an enormous assumption. Many babies are fussy, colicky, or otherwise high maintenance. The variation at this age is just ridiculous.
I think this varies enormously between kids, and to some extent between parenting styles as well. With my particular kids (5y and 3y) I can often spend a whole afternoon working on a project while they play individually or together; at younger ages the time commitment was far larger (nearly continuous).
And when it comes to "it depends on the kid", oh dear. I try to practice stoicism whenever I remember about it, and logically I try to remember that I should be grateful for his fussiness and rebelliousness, since they give me occasions to practice my stoicism. But it sure does feel a bit like stockholm syndrome sometimes.
My personal experience both from growing up and now raising three kids is that the kids will accept just as much independence as you are willing to give them.
Though they do get to be whiny sometimes. And I absolutely agree about the relentlessness of it all.
I wouldn’t describe anything (after maybe the birth itself, which I can’t answer about) related to our 6 month old so far as “difficult”. It is only “physically taxing” insofar as it takes a lot of time every day, and alongside other obligations (in particular, the 3-year-old) that leaves everyone a bit sleep deprived. I would say the 3-year-old takes at least twice as much time and 5x as much effort. (The work is stuff like reading books together, building block towers, running around the playground, preparing food, etc. Nothing especially challenging by itself.)
My wife gets burned out pretty quickly when I leave her with both kids on the weekend for most of a day. She’s generally relieved to get back to the office on Monday and leave them to me. :-)
Not sure what kind of beings you're raising but my experience with three kids is that it gets progressively harder until around 5 and stays stable until 13 and gets much harder from there.
If it helps any, my experience is that it's hardest on the day your child is born, but it gets progressively easier as the years go by and the children become increasingly self-sufficient.