We have three and a forth one on the way (which was not planned btw)
We had a boy and a girl too and then another girl. They are 5, nearly 3 and nearly 1 and although the first couple of months were stressful, it is getting easier every day.
The older two were always very sweet with the baby and care a lot about her. If you go for state schools, I don't think that money is such a big issue, especially as you can hand down all clothes, toys, etc. Travelling, of course, gets more expensive - so it really depends how much (air) travel you have done so far and are planning in the future.
I always wanted three and it's just so much fun seeing the three together in the bath tub or playing together.
But if you can, get some help in the beginning, especially the first three, four month can be quite tough.
I wish you all the best in your decision whatever it will be.