My twins were 17 months when my third was born. The great thing with them being young is that they are really adaptable and won't bat an eyelid at having another baby appear in the house. We barely got out the door for the first few months (going out to the garden was an outing!).
I had no help so put the twins in nursery two days a week. In hindsight I wouldn't have done that, I thought having them out of the house would be easier but it would have been better to get a nanny who would take them to an activity for a couple of hours so get them out but also then take the baby so I could have had time with them.
I won't lie, it was incredibly hard but DS3 was a Velcro baby and a terrible sleeper. I expected to be able to spend time with the twins but he never napped in his cot so I always had to be carrying him.
DH really stepped up and I couldn't have got through it without a lot of help from him.
Breastfeeding was a godsend, I wouldn't have been able to juggle bottles as well (didn't manage to b/f the first two.
They are now 6 and 4 and its so much easier and they are all so close, plus they tend to like doing the same things. They really are lovely together (when they're not fighting). I see mums at school dragging a school child and toddler and baby around and it looks so hard. I do hope that for them having such a close bond will outweigh any disadvantages. DT1 had reflux which was never fully resolved and we've had a lot of issues with him over the years, basically attention seeking behaviour so I think he has found having so many demands on our attention the hardest.
I'm not sure if that will help or not, I feel its fairly negative but it was a really difficult time for us. Please let me know if you have any questions 