I found the jump from 2-3 to be the easiest of all actually. 0-1 was hard cos it was our first and we didn't know what to expect, and 1-2 was just generally difficult because DD was so used to being the only one, and suddenly I found myself having to sometimes put her needs second to the baby's which was frustrating for her and for me, and I had bad baby blues (I suspect I had PND actually) which didn't help at all.
But 2-3 was easy. I was used to juggling more than one child already, and he just seemed to slot right into the family really easily.