Lots of communication is generally what makes things work for us. If someone is feeling left out or neglected, we make sure that's discussed and addressed. Sometimes it's as simple to fix as rotating who's in the center when we cuddle a little more often. We also have cute nicknames for each other, compliment each other frequently, and sometimes two of us will team up to plan an outing or make a gift for the third.
The other big thing that makes everyone feel special is that we acknowledge and honor our differences. Sometimes I can relate more to one partner on a certain subject, but a different subject is far easier to discuss with the other. We all understand and appreciate the way we balance each other's personalities and support each other in very personal ways.