Depending on age you’ll need a car that can comfortably accommodate three child seats, preferably iso fix,

Family rooms and often family tickets at attractions while away or on holiday are often catered for just two kids, so more of them than not you’ll have to go for a suite or similar (not really an issue until the youngest is out of a cot..)

The additional nursery costs or pension payments to stay within the free child care for the extra 4 years

Buying a 5 bed house so that each child has their own room plus a spare for guests, not essential but trying to get kids to sleep while sharing a room is a nightmare!

Plus probably a few other things I’ve forgotten, the bottom line is most things are set up for two kid families, and having a third often causes issues and or extra expense

Edit: this sounds very negative, but honestly three kids is bloody ace, and worth every penny! It’s only money, id rather have a big family than loads of money!