I’m the eldest of three, with a narrow age gap between me and Sibling 2, and a bigger gap between siblings 2 and 3.
We would have been a better balanced family if we’d been four children (which was what my parents actually intended). The problem with three is that it divides naturally into 2 + 1, and my experience was that someone was usually left out (and because of different interests and temperaments, that someone was usually me).
Benefits for me: I love having more than one sibling, including of both sexes - very different relationships with them.
Disadvantages: As the eldest, it wasn’t really very convenient for me to show need (eg emotionally). I was definitely expected to get on with it as the eldest. This has created some quite damaging behaviour patterns both for me and more widely within the family.
Money was tight growing up, although not to the extent that we didn’t have what we needed - but I was in my teens before I had my own bedroom and that really mattered to me. I would have been happier with my own space long before then.
These days, car seats and holidays (and even daft things like packet sizes for stuff like a box of lollies from the supermarket, etc) would be factors for me. As well as the cost of childcare and my ability to meet all my children’s needs.
DH and I have two and will definitely not be having three.
If you want three, and feel confident you can provide for them all, then go for it - but go into it with your eyes open.