Apparently research shows 3 years to be a good gap for both siblings. The older one has had a lot of attention but is now more independent and the younger sibling is able to have more attention than if the older sibling was younger, ITMS.

I think it also depends on other issues, like personalities or needs of each child.

We're unlikely to have another child, but if we do, I wouldn't start trying until DS3 is 18-24 months, even with our history and only 1 tube.