Can I post as a Mum of three who are now well beyond the baby stage (ages 14,12 & 9) ...most days!
I actually found my 3rd the easiest baby, my oldest was already at school and my middle son at playgroup so the routine that I had struggled so hard to find with the first sort of happened naturally, although my daughter did come to school in her pyjamas on more than one occasion.
The baby years are tough but they do pass relatively quickly and it may help your decision if you think beyond those baby years and especially about your aspirations for your children and your family.
First and foremost do you want to work and if so how would you cope with childcare? I never planned to stop working but it sort of happened and it made sense to have a break with my third as we couldnt have afforded or wanted a nanny to look after them all.
But money was tight, eg £30 a week for school dinner moneys seemed extravagant - I could make packed lunches for a lot less. Birthday parties were more homespun and we relied on generous Grandparents to help with Christmas.
When my daughter started in Reception I was lucky to find a part-time job and this made life financially a lot more comfortable.
We still can't afford private school for 3 but we have been fortunate that they are all at 'outstanding' state schools. We have had to keep a close eye on their education, help them at home, boost with tutoring and fill in the gaps (eg competitive sport). This involves 2x football matches, 2x rugby matches/training, swimming and hockey every weekend which as you can imagine leaves time for little else.
We didnt go on foreign holidays at the start - caravans in Devon, staying with Grandparents etc. Also be aware very few hotels in this country or abroad offer family rooms for 5 (so you will always need 2 rooms & may have to split as adults if rooms aren't interconnecting). We now can afford one really good Neilson sailing holiday a year (my treat as I dont have to worry about feeding/entertaining/washing clothes etc & we all love sailing/tennis being outside) but still we look at the price for 5 of us and think, one of the kids is going to be on a camp bed.... and it's one school trip a year as a rule.
High value items like bikes are handed down - my 12-year-old will probably get his first 'new' bike this Xmas but he doesn't mind - he loves having his brothers pre-loved stuff.
We don't have a big car & its 13 years old with a roof box and not much leg room now the kids are getting bigger but we get by with bikes and a Family Rail card. We also are reliant on friends to lift share when matches/activities clash but I think/hope we reciprocate enough in return.
On the plus side, my oldest is incredibly patient and good with his little sister and her friends. My daughter is very mature and considerate and adores her big brothers and their friends. Interestingly the boys' friends who have stayed the course have been the ones who were most accepting of having a feisty young sister insist on joining their games, doing their hair and nails etc. And I can only recall one tiny little girl being so overwhelmed by the big brothers that she didn't survive the play date!
They can share, take turns and are considerate towards others (again most days!). When my oldest started his moody teenage phase his younger brother and sister shrugged and played together, and occasionally reminded him that he could still have fun with them.
If you do go for a third my one bit of advice would be make sure you have a big kitchen table - many of our happiest memories are as a family on Sunday evening having a roast or when I somehow mucked up play dates and ended up with 6 or more children for tea.
So apologies this post is longer than I meant it to be but as I started it is back to aspirations: if you are happy to slow down a bit & make packed lunches, fill your life with occasional treats rather than 5* hotels etc, from my experience 3 kids has made a rich family life. If you think you would rather concentrate on the two lovely children you already have, then stick with that and be grateful.
We are incredibly lucky to have the choice and we need to keep remembering that as we live in this fantastic South London bubble - our kids are well fed& clothed, well educated and generally very happy, as well as loved. Not all children in the world or even in other parts of South London are so fortunate.