It depends what level of difficulty you want to live life on. 3 is far more work and for not much more reward than 2
3 is far more work, that's true, but the 'reward' is immeasurable. For me, having three is perfect - hard, hard work, and so very expensive, especially the teen years, but perfect for our family. I really can't imagine my life without having had my third child.
I found the jump from 2 to 3 to be massive - I had 3 kids in 4 years, I was 36 when I had my last, and it seemed to be a bigger jump than even having my first was. I guess I had mentally prepared myself for the first, but I'd naively assumed that having a third wasn't much more work than two - boy, was I wrong!
They were (and are still) all good, fairly well behaved kids, who have never given us much cause for concern. But it was vey difficult in those early years, having two pre-schoolers while also having a baby. I needed eyes in the back of my head in a way I never needed with two. And you never get a minute of peace, not ever! The bickering with three of them is easily three times the bickering of two. There's three relationships there instead of one. If you've settled two of them, the third still needs attention. I look back now and I wonder where I got the energy to manage everything that needed doing without going crazy! Sometimes I would look at my sister, who had two children of a similar age, and think how very easy it looked.
As they grew it got easier, but ferrying three kids to different clubs is a full time job on it's own. Plus giving individual attention is harder. But not impossible.
I was lucky that I worked from home around the kids, and have a husband who didn't work long hours, so we didn't need paid childcare. So the early years weren't all that expensive. Which was lucky as we didn't have much money anyway! The biggest expenses were the larger car (definitely recommend a 7 seater, mostly for the large boot, and ability to take extra people, friend, nan etc) and holidays. If staying in hotels, holidays cost usually double the cost of a family of four. So we mainly camped, or did self catering where we could all squeeze in to a two bed apartment.
But the teen years were something else! Phones, clothes, pocket money, school trips, holidays, eating out (effectively with 5 adults), driving lessons, clubs etc all quickly add up. And then there's uni costs!
However, you just cut your cloth. My DC learnt that they couldn't have everything on a plate. Things like phones and expensive school trips and driving lessons were either Christmas/birthday presents, or they helped to contribute to these things themselves. First cars they had to buy themselves, with a small contribution from us. They all got part-time jobs from 15/16, to help afford these things.
I'm very proud of how they've grown up. They are thoughtful, kind, un-spoilt individuals, who work hard for what they want.
Three is harder, and more expensive. But it's worth every effort and every penny. It's amazing, and I wouldn't change it for the world.