My friend has twins via IVF and then became pregnant naturally when they were 6 month old, there is 15 months between her 3.
I was astonished how well she coped but she just kind of 'let go' of all the things that we think matter (cleaning etc) and went with the flow for the first 3 years. Her house was trashed, which is hard to imagine when you see it now! I asked her once how she did it (I was in awe, no family with 200 miles and DH works long long hours, she was basically on her own) and she told me that she always told herself two things - having all three within fifteen months was never ever as stressful as trying to have children and not being able to, which they did for nearly 5 years before the IVF worked. And also that 'everything is just a stage, this will pass, this will pass'
In terms of discipline, she picked her battles and focussed on the one that was important to her at that stage, got that right and moved onto something else. So, she might ensure the children always said 'please' and 'thank you' but not pick up on them using their fingers to eat and smearing ketchup all over their plates (bad example, but you get the picture!) until she was ready to tackle this.
I think the hardest thing she found was the stage before the twins were old enough to walk anywhere far and parks etc, as she couldn't be in three places at once. So, they have a large garden which they used a lot until she was ready to cope further afield.
I probably shouldn't say this, but I read some posts that people make about behaviour etc and following everthing through always and I would love to put them in my friends shoes for a week at that stage of her life!!
Her three children are not 12 and (nearly) 11 and are delightful, well mannered, beautiful children, who are exceptionally close.
Sorry - waffled a bit, I am multi tasking!