There are a few practicalities changing from 2 to 3 which crop up. I have three and some of these subtle things irritate. I'm not knocking three, I love them but if you want to balance desire and practicality they may be something to consider.
Car seats are a bit of a challenge when you need three across a back seat. You can get ones that physically fit, but its a juggle to get them in.
Flights as more than one child per adult can be a bit more of an ordeal. Carrying luggage an adult can manage two bags or a bag and a child but one adult needs 2 children, the other 1 and that leaves one free hand for the families luggage.
Hotel rooms, cruise cabins, free child places all tend to be based on families of 4. We went on a couple of family cruises with wider family but needed two cabins as we are a family of five. First two places full price then 3rd and 4th in cabin almost free. It ment we essentially paid double the price to have our then toddler DD with us.
Likewise hotel rooms. Its not just the cost, its the convenience. Great if you can get interconecting rooms, okay if you can get side by side rooms but if you have to get two rooms with one adult staying in each it can detract from adult time. We find that larger family rooms tend to be located in one area of a hotel so the 3 person family room can be on a different level to the 2 person room - sometimes meaning we need to pay for 2 family rooms and request proximity to holiday together.
For complex additional need reasons I've had spells of three children at three schools, it's logistically hard work. With three, as they grow you're mum and dad taxi. If you produce little socialites you may sacrifice every evening to the logistics task of three sets of different drop offs and pick ups at clubs.
None of the above are essential. You don't need to use a car often, go on holiday, let your children explore lots of different/ individual after school pursuits. Compromise is a great life learning tool and with three, it's one that mine are mastering.