Hey Joline! I often wonder how you moms of 3 do things, it sounds really difficult. Although I'm not in exactly the same boat, I can still relate, as I wonder how to meet my twins' needs.
The thing I've noticed is, they're not the same. It's enough to make a person crazy, but it's also a blessing.
As an example, I used to rock my dd every night, while ds fell asleep in the crib (the rocking chair was in their room). When they invariably woke up, I'd take one or both of them into bed w/us, but for putting them to sleep the crib was essential. And my dd absolutely had to be rocked to sleep, every single night. Ds fell asleep on his own without a struggle, sometimes watching me rock his sister.
I felt somewhat guilty about this, until I noticed that my ds was getting his needs met differently -- that boy has to have a good tickling roughhouse at least once a day. I play w/dd too, obviously, and even moreso now that they're a physical 18 mos; but ds needs his special attention.
I'm willing to bet that you do a lot w/each of your children to meet their individual needs. Your 3-yr-old probably needs that cuddling time alone w/mommy; I can just imagine the extra attention your adorable twins get from strangers every time you leave the house. As a first-born, I can tell you the pressure on the oldest child is rough (check out "The Birth Order Book" by Kevin Leman).
By keeping your relationship with your oldest strong, I'd bet you're making the best choice. If he's insecure, that can throw everyone out of whack while he demands even more attention. Are you planning to enroll him in preschool?