I have 4, with an 11-year gap, first to last.
I second the suggestion on family walks to build stamina before you go. Your 5yo may need a stroller, at least some of the time, specifically in Epcot.
Dh and I split up a lot. Typically, we spend mornings doing "little kid" stuff, then after lunch, one of us takes the younger two (6 and 3, currently) back to the room, while the pther parent takes the older two (14 and 12) on the thrill rides. We meet up again later. The 6yo may not nap, but she'll need a break from the sun.
We also split other duties. DH is our "fastpass runner"--when he's hoofing it for that, I do a bathroom break with the kids. We designate a specific waiting spot outside the bathroom where we all meet up again.
When we get CS meals, we all order, then one of us finds a table, gets little guys set, etc. while the older two help carry food and get condiments.
Really, it's not difficult, but it requires teamwork and a little bit of effort. In addition, I always schedule at least one special trip thing for each child. For my oldest, it might be shopping at DTD, just her and Mom, sampling at Ghiradelli and sniffing our way through Basin. For DS3, we have Monorail Day--we'll ride around, eat at Chef Mickey's, pool-hop (we have DVC). That's the only thing planned for the day. The older kids are fine with this because they get their special things, too. I also find that, since my older ones get their thrill rides, shopping, Jedi training, etc., that they're much more tolerant of their younger siblings' needs. So, they'll patiently sit through Dumbo, secure that after lunch, it's Splash Mountain for them.