I'm sorry for your loss @ItsAllBlahBlahBlah. 
I have 3 older kids, youngest in mid teens.
The food they eat is significant! Holidays etc become very expensive, and the reality is that we haven't had many big holidays compared to friends simply because we couldn't afford it.
Cost of clothes etc obviously increases, and instead of 3 pairs of Clarks school shoes at £30 each, you're looking at £70each for a pair of Nikes for school, the same for out of school, and the same for sports and PE kits. Then they grow and they need replacing.
But that's only money..if you've got it, great, Not a problem.
For me, the real impact has been on me. Mentally. And career wise. The career thing was an active choice. I wanted to care for my kids (and couldn't afford childcare!) so we muddled through, dh working days and me working evenings and weekends. That was pretty exhausting. I've got a job now that I'm happy in and pays ok, but not compared to other women my age who have SIGNIFICANTLY higher incomes than I do. Having a third child made that last a few years longer.
Mentally, the load of 3 kids has just about broken me!
Problems in pre teens are generally easily sorted. As they've got older, life has become more complicated. There is never a time when all 3 are on an even keel... mental health, broken hearts, alcohol, sexuality, friends, bullying, self esteem, boundaries, education, all sorts of things happen and need supporting.
On saying all that, I wouldn't be without my 3 kids, and I really did enjoy having number 3... you know what you're doing, and are somehow more confident in parenting by that stage. The dynamic of 3 is great, and when we go anywhere as less than a 5, it feels so neat and tidy. Now they're older, they still get on really well, and it's lovely to see their relationship continue to grow and change as they become young adults.
That one extra does change everything. 