It's hard to get an idea just from pictures, but the rabbits all look very thin. Even the fur on the brown one who is still alive in that last photo isn't looking good either.
I'd say that you're looking at malnourishment, first off. So start by looking at their diet. Do they always have food available? Rabbits eat throughout the day and night, so they should always have something to eat and drink on hand. Their main nutrition comes from grasses or hay. I see you have rack bottoms, so they could be losing food through them. Also bear in mind that rabbits are more sensitive than us to chemicals like pesticides on foods, so make sure what you're giving them is chemical free. Have you gotten foods which could have had excessive pesticide treatment? It will interfere with their digestive system. To many low fibre foods will too. Are their poops soft or firm? They should be firm unless it's a cecal poop.
If they're getting plenty of food then they could have parasitic, intestinal worms. It's not very likely, however, considering that they are un under cover and have the mesh bottom for their poo to fall through, making it less likely that they would infect others. It's possible, though.
Check their fur and skin and see if they have any irritation, lice or mites. The fur condition could be caused by that, but low nutrition will cause that too.
You're so right, rabbits aren't easy and will go downhill quickly, so you'll only have a short time frame to catch something and put it right.
I'm keeping all my finger crossed for your rabbits. Please feel free to ask any questions and keep us up to date.
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