I have b/g twins too, and their younger brother was born exactly 3 years after the twins' due date. ( They were born 11 weeks early though)
I found it was a lot easier the 2nd time round, I can clearly remember thinking 'Thank goodness I don't have to change another nappy/ feed another baby now!' I luxuriated in the intimacy of just having one baby to cuddle, after spending a lot of time feeling guilty about not giving each twin all the time and cuddles they needed.
As for the twins' reaction, they were wonderfrul, very loving towards the baby, with no jealousy shown. I think it may have been partly because they never had me all to themselves, and were used to sharing me.
The twins are now 6 years old, and their brother is 3. We are unschooling all of them, and really enjoy their company. They all get along really well ( most of the time!) and I don't find the twins shut him out of any play, nor that they are closer to each other emotionally than to their brother.
Of course, you still get the sleepless nights, the terrible days when nothing goes right, but, looking back, I wouldn't have it any other way.