First of all congratulations! I think it's great to have 3 so close in age.
My dc was 8 months when I got pregnant with twins. They were supposed to be 17mths apart, but ended up with an even smaller age gap of 15mths.
I agree with what previous posters said, so not repeating what they've already posted. Just some additional thoughts:
As you know, with a twin pregnancy there is a higher possibility that doctors will tell you to take it easy od even go on bed rest. Not saying that this will apply to you and not wanting to sound negative, but I just found for myself that it helped to be prepared in good time. If you can introduce soeone as an additional carer for your dc1 early on, you have the peace of mind that he/she will be happy and well looked after if you can't do it all yourself. It minimizes everyone's stress levels.
Once the twins were born, I found to my surprise that I actually didn't so much need someone to look after dc1, but rather someone who couldtake care of the twins (and the household...), since dc1 really needed mummy a lot, while the twins didn't really mind whether it was daddy, babysitter, or granny feeding and cuddling them. And this brings me to the next point.
Do not let anyone, including yourself, pressure you into breastfeeding at all costs. It's great if it works for you, there are a lot of women who manage to breastfeed their twins and I'm in awe of them. But this is a time where you need to be pregmatic. It's hard enough with one baby, even harder with two! Add to the equation a very young dc1 who is almost a baby him/herself, you might want to consider freeing up as much as possible of your time for dc1.
Mine are still little and I can only base my opinion on our first year. But I find that if gets easier nowbthat the twins begin to walk and the weather gets warmer.
Congratulations again!