i chose 3 years between 1 & 2 coz my MIL's youngest were 3 years apart. They were 4 & 7 when I met them & got on really well.
On top of that I wanted 1 toilet trained & sleeping well in her own bed before I had 2.
Between 2 & 3 I raelly only chose 3 years again because I wanted the same for them all.
DD1 was alot more grown up mentally than DD2 though & handled it alot better. DD2 is still a bit out of sorts, but she's getting there.