I have 3... they are now 14, 15, and 16. Going from no kids to 1 kid was a huge change. One to two was a different kind of change... I kept wondering if I could love another baby as much as I loved the first and worried like crazy about it till she got here and found that YES I could!
Once we had 3 kids, we'd tell our friends, "sure, drop your child off with us for a few hours. once you have 3 you might as well have 6!"
Three is so much fun, and so unmangeable at the same time. You learn a lot about prioritizing.... not just chores, but what REALLY matters in life. So I think the more kids the better. Just make time with each of them alone on a regular basis.
I think the sooner the better, because I hated being the older sister in my own family that had to take care of my bratty little brother. That's why I had mine close together. It sucks being stuck as the sitter-sister.
Fortunately, as my brother got older (like college)we bonded.
Good luck!