I got pregnant with little sister when my boys turned two. Husband didn't prefer to have the kids far apart in age, and felt the near 3 year spacing was ideal. I agreed.
Some thoughts: I really felt like I didn't have the "courage" to get out with the boys alone right when they were at the more flexible 2's. I was burdened with the pregnancy and spent a lot of time at home, or the library. But we didn't venture out to the museum, or zoo, much alone. I think I would have been more adventureous if I hadn't been pregnant.
The boys were potty training right when when I was getting very large with the pregnancy. I wanted them out of diapers before the baby came, and that didn't quite happen.
On the other hand, they boys still rested/napped when I was needing naps with the pregnancy. WHile it was difficult to have 3 under 3 for the first few months, they are at the perfect age spacing now. They play well together, and the boys really don't remember a time without little sister. We have almost zero sibling rivalry.
I did need someone to help with the boys when I came home with little sister for the first few days (I was recovering from a c-section).
In a "make your life easier" kind of way, if you don't have a great twin stroller, invest in one. I purchased a jogging twin stroller with an optional swivel wheel position, and it made all the difference in my freedom. I could actually walk for exercise, push the twins and sling the baby. It was the best twin purchase I'd made in years. I'd had a twin umbrella stroller, but this actually made exercise possible agian.
And on a personal note, I knew when my boys were born that I wanted another baby. I wanted a single baby experience if possible. It was really nice having just one-- nursing was easy, and much more flexible. I could actually co-sleep without taking turns with who was in the bed. It was such a different experience than the twins, which was a lot of fun, too: just very different.