We adopted our older two together , then their birth baby sister a few years later, once the older two were at school.
Looking back on the early days it was a lot easier with "one at a time".
I found dividing my time when the first two came home difficult as the older one, two years old, was very demanding, rightly so. Fortunately her sister was just a baby, six months old, so I just "wore" her and she was constantly with me.
When we adopted dd3 , as a baby I found it was like having just one child, during the school day anyway!
I enjoyed dd3 early years a lot more, as I felt I had more time with her on a one to one basis.
The bond my older two have with each other is wonderful to see, and I'm pleased that we were able to make that happen.
As others have said, I think it depends a lot on the children.
To be honest though, if you are having any doubts over a sibling group, you would be better off with just one child.
Best wishes.