It's hard. That's no lie; and I only have one older kid. DD was 2.5 and at a very difficult time of her life when the twins came, and we had some very rough spots. We dealt with fear and anxiety from her-- what if I need mama, and mama can't help me because she's busy with the babies? We dealt with jealousy, and acting out from jealousy. She acquired a few nervous habits that we had to be patient in dealing with. And once the twins got mobile, we dealt with bickering and fighting and constant battles over somebody "messing up" somebody else's game or toy. Also, DD had to learn to do a lot of things for herself, and she resisted that a LOT.
I think you just have to take it minute by minute, take care of the one who needs you most urgently first, and every so often step back and take stock and see if there are any needs you're not addressing. Arranging a special time of day for the older child to be alone with each parent is nice if you can manage it-- even 15 minutes is the WORLD to a small child. When you can't, you just do the best you can. I remember teaching DD1 to hold her own book open while I read, because I had my hands full of nursing twins and couldn't hold the book.
The hardest part is the sleep. If you have just infant twins, you can nap when they sleep, but with an older kid, you can't just go to sleep in the middle of the day, and the lack of sleep can REALLY wear you down. Preschool is great for that, as is a helpful grandma or neighbor or friend who'll come and play with the child for a few hours so you can take a snooze with the babies.
And yeah, preschool helped a LOT. It was time for me to bond with the twins, so that I felt okay about spending more time with DD while she was home. It was something DD could do that was JUST FOR HER. And it was a chance for her to escape the general level of anxiety that sometimes exists in a house with multiple newborns in it!
It also really helps to be careful about the relationship you have with your partner. When DS was colicky right in the middle of DD's potty training, and all of us were not sleeping at all, it was easy for me and DH to lapse into being short with each other and bickering, and it was THAT that stressed DD out more than anything directly related to the babies. Watch your general anxiety level and the way you and your partner are dealing with it! Make sure that if you wind up arguing or something in front of the older kid, that you "make up" the argument in front of your child too, so that he can see that his family really is safe and stable.