Hello! No real answers here but really interested to see the responses. I have two children, the youngest one is brand new. I would love another, but I also think I may consequently kiss goodbye to my nascent career. I am an early stage academic, currently doing a postdoc, but I am not that 'young' (37). Part of my problem would be timing - I would need to have another one relatively soon, which would cause real problems with getting a job (essentially applying whilst pregnant, not good, or getting pregnant if and when I got a job) and possibly necessitate some time out. In asking myself the same question, I find myself constantly asking whether my career is more important to me than extending my family - ie I seem to assume that there will be a choice to be made if I have another one, and that the compromise might be that I have a job rather than a career.
Anyway, from looking at friends and family, I think that holding down a full-time career and having three is significantly more difficult than two and certainly quite exhausting. It's much easier of course if you earn a lot and can afford a nanny for example, or something similar. My sister's experience with three would suggest that it might be slightly easier if you leave a bigger gap (in her case three years) between the last two. But I would say that having three has certainly had a negative effect on her career. Not sure if it's temporary or not - she's a journalist. Her's are 8, 6 and 3 now. I hate to say this, but now that her oldest two are at school, it is probably harder, as not only is there the after school issue, but also SO much homework to supervise, and after school activities to co-ordinate etc etc. And with three of different ages, there's different pick-ups and schedules too. Having said that, they have a lovely, noisy, big family life which I envy.
Also, this shouldn't matter, but I think that people are far less sympathetic about childcare difficulties if you have more than two. I think that two is considered 'normal', almost a right, but with three the perception is that you have chosen to make your own life difficult and should suffer the consequences. Of course, this seems to only apply to mothers not fathers!
Sorry, a bit of a ramble, but this struck a chord with me as I am pondering the very same thing!