My take. Our twins were a ton of work. If you've only had multiples to take care of, you don't know what it's like to just take care of one child. So while it was a lot of work raising twin babies, we just took it as the norm. When we just had our 3rd child, we said to ourselves that it's a piece of cake. Feeding, diapering, burping, etc. a singleton is cake compared to doing it for two at once. The hard part right now is working the schedule of feeding the newborn every 3 hours and fitting that in with the twin toddlers' schedule. The afternoon nap that my wife used to take is now gone and 3 am feeding is now added for her. We marvel at how easy it must be to have just a single child.
We've been luck to have two boys that behave very well for the most part. We've worked to teach them to only color on on their color books/paper and not to write on walls/furniture. When one is trained, the other usually follows. So we just have to work at getting either one to understand. Also, they're a bit over 2 1/2 yrs old now and talk to each other all the time. They play with each other too so that helps to entertain themselves. Thank goodness that they have also been really good and respectful with their new sister. They are understanding when we are feeding her and have to do things for her.
So for us, we wanted another child after our twins. We also didn't want them too far apart or else we felt that they wouldn't be close. I feel bad that my twin brother and I left our younger brother out during our childhood. My wife feels that her older sister and younger brother aren't as close as they could be also due to age. So our twins and daughter are about 2 1/2 yrs apart. Not too close, but not too far in age. Hopefully the twin boys will be body guards for our daughter.