Usual order:
Salt and chilli ribs
Seaweed
Salt and chilli chips
Rice X 2
Special chow mein
Lemon chicken
Sticky chilli beef
Beef and black bean sauce
Again, that's 4 mains and 5 sides for only 3 people?
I don't really count rice or chips as "side". When you have a main meal at home, do you just have meat in a sauce with nothing else? Usually you'd have carbs or veg with it too, so the rice/chips count as a main to me alongside the main dish.
So lemon chicken with rice is a main. You just have to order the rice as a "side" but it's not really a side dish as in "extra".
So I'd look at that order and think 4 mains (chips and rice being includes alongside the mains because why would you just have lemon chicken on its own? You don't order curry and not have rice....), 1 side (seaweed), and 1 sauce.
People also order multiple mains because they don't eat the whole thing. They might take some sweet and sour chicken from the container, then some chilli beef, for example. So a lot of people share the mains meat/fish/veg between them, rather than eating it to themselves.
Even just for myself I'd quite often order two meat dishes because I won't eat a whole one. So I'll order a sticky beef and a sweet and sour, plus rice, but I'm only putting some of the sticky beef on my plate alongside some of the sweet and sour, not the whole things! Then save the rest.