I knew before I clicked on that thread how it would go. MN sure loves its two child policy, guess we should put posters on this thread in charge of China.
If you wanna complain about the growing human population do it in a country where the fertility rate is 8 or 9, not in the UK...
While OP's finances is definitely something she needs to consider, frankly children don't actually need nearly as much stuff as society (and big brands !) would have you believe. Also a third would certainly be using a lot of things OP already has - car seat, pushchair, sibling's clothes, etc. Sure, a third needs and should have new things too, but a third doesn't cost nearly as much as a first.
Most cars seat 5 people, unless all three kids need a car seat.
Sharing a room with a sibling really isn't as awful as MN makes it out to be. In fact I don't get the "each child MUST have their own room or else" mentality. I shared with my brother until I was 14 and only stopped sharing because my other brother left and freed up his room. Oddly enough I survived with my sanity intact.
As for uni costs, depends where you live. I'm in a country where uni is virtually free so not concerned about that. Many young people manage well with student loans and a part time job.
As for having enough time for three, my parents managed well enough. Sometimes I was quite glad that they were too busy with my brothers to pay too much attention to me, it gave me some breathing space. Frankly I feel bad for kids whose parents are all over them all the time.
I don't think there's a right or wrong number of kids. I think people are heavily influenced by the experiences they have had as children, positive or negative. People on here seem to be also influenced by the politically correct, virtue-signalling two child policy. I think people who stop at two do so because they only wanted two to begin with or because they truly couldn't afford more (or valued their comfort of living more than a third child, which is totally fine). I think anyone who says otherwise is lying, possibly to themselves too.
OP, your big mistake was asking here. Don't. Think about it, about the costs and logistics, imagine you had three today and how you would handle it practically, and then make your decision.
The funny thing is that if you'd come on here saying you were pregnant by accident with a third and not sure whether to abort or keep it, people would be tripping over themselves to reassure you that you'll be fine with three. Cognitive dissonance at its finest.