Five years is often old enough to (mostly) stay seated, (sometimes) for up to half an hour. At two and three, a dinnertime without playing, wiggling, and messing around with food would be a miracle. Impulse control is very hard to come by in toddlers, and they can't help it.
Two hours at the table is as grueling to your kids as it is to you. I've never had to share dinner with more than one kid at a time, and even with 3 adults tag-teaming, it can be hard to get much food into the small person.
Traditional meals-at-table are simply not "normal," in the sense that before agriculture made nomadic life unneccesary, humans were largely grazers – eating what they could find when they could find it. And they were far hungrier than we are today. But even then, kids probably relished the occasional huge feast of antelope for approximately 3 minutes before running off to play. And small children today are still pretty primitive.
Rather than having every meal become a protracted battle, you might want to accept reality and see if it works better. Focus on getting your 5yo to practice manners for 10-15 minutes for 1-2 meals per day for starters, and give the toddlers the opportunity to graze during the day on little bowls of healthy foods, bites of cut up fruits and veggies, omelet, whole-grain crackers or cereal, cheese, a few bites of yogurt, etc. Studies show that when offered a range of healthy foods, kids are good at selecting a pretty well-rounded menu, and while they don't eat the same amount every day and may have no appetite at all during some traditional meal-times, they do fine.