We have three who are DS1 13, DD1 10 and DD2 7. I knew I wanted a third as soon as the second was born and tried to talk myself out of it but the heart wanted what the heart wanted! In all honesty it was tough from the pregnancy onwards (felt like my body was at it's limit by the third time (aged 38) and I had two younger ones who still needed full-on care) and hasn't stopped. I wasn't prepared for pregnancy loneliness as all my mum friends were done at two and I didn't have the same social circle and excitement about number three - honestly a few of them felt sorry for me/thought I was very weird. Looking after three tinies is truly knackering. I thought it would get easier when the third started nursery but that got delayed thanks to the pandemic and I don't think I have ever really recovered from trying to look after her while homeschooling the older two. I know the pandemic was totally unexpected to us all but it was a lesson that life does throw up unexpected changes and it's a lot harder to pivot or adapt when you have more dependents.
I do feel like the older kids have missed out on our time and opportunities because of the youngest. My relationship with the oldest in particular became more distant right away which has been very sad for me (he is a boy and just switched at age 6 from being mummy's little boy to a total daddy's boy - probably was going to happen anyway?! Who knows! Now he's a teenager he pretty much just grunts at me but has a close relationship with his dad I'm a bit jealous of. Another story!). Every holiday and trip and dinnertime conversation, family movie viewing etc has to be orientated to the needs of the littlest. When we go out as a five we nearly always have to split up to go at two different paces - you expect this with a baby but it still happens long after we ditched the buggy because they all have different needs and interests. (I have seen this happen with families who have two kids with a big age gap though too so not necessarily a multiple kid thing perhaps). This could just be our family though - some people make the youngest keep pace with energetic older kids and adult interests, but either way I think someone loses out. Our marriage has definitely been put under huge pressure extending the sleepless baby/toddler period, finding it harder to get babysitters, having to split up on days out so less time together, financial pressures of buying a bigger car and house. We are ok but I do wonder every day how much easier it would be if we hadn't had number 3.
That said, OF COURSE DD3 is a joy of a human being, a child I am utterly blessed to be the mother of, whom I cannot imagine not having in our lives. I appreciate her littleness in ways I didn't quite with the youngest. She has definitely reaped many benefits from being the youngest and I think the oldest has too. Middle child is hard but I can see how she is developing in ways that will help her in the future. I do think having to compromise and learn to get along and go without things has been good for them all. The good times are exceptionally good when everyone is on form.
My advice to anyone thinking of extended their family is to 'stress-test' your life - be really honest about the areas that you do well in as a family, and where things could be better. Another baby will exacerbate the weak areas and for some families that is not tolerable. The added pressures and complexity of three kids do not go away when the youngest gets to sleeping through the night/nursery/school/whatever stage, they just evolve into new pressures and complexity. I don't think I did all this enough, I kind of waved my hand and said 'it'll be fine - other people do it'. Do NOT do this, especially if 'other people' are celebrities or people whose lives you don't see up close and real. Have counselling if necessary to really address why you want a third - is there something missing? Are you just grieving the end of baby days and not ready to move on from that yet? Did you have difficult births/early circumstances with the first two and yearn to put these 'right' once and for all? All of this was true for me and I truly wish I'd explored it properly with a therapist before getting pregnant.
Good luck, OP, and anyone else in the same situation. (And to anyone with more kids than I do who is laughing at how easy we have it with only three - I bow down to you!) xx