Awww, I'm so sorry Flo for the loss. I know it's not abnormal to have runts that may not survive, but I know it's hard to know if they could have if some intervention was given. Because of some other things going on, I will be seeing the vet tomorrow, but I know that may be too late.
I don't know where you live, but do you have any rabbit rescues or rabbit savvy vets near you?
When I search around the internet, I do get conflicting information - like supplement the mother's milk with a special mixture, but then others say that you have to be so very careful because you can actually suffocate them doing this. So I think it's best to find a vet professional who knows about rabbits. And even a breeder may be able to help guide you.
You can also ask on Etherbun http://pets.groups.yahoo.com/group/etherbun/
, I did a quick search for answers, but it may be better for you to ask you specific question. They are a get down to business type of forum which is great for urgent medical concerns.
I just went to the House Rabbit Society chatroom, and got someone who also advised you speak to a vet since this is your first time.
I'll send healthy vibes the third struggling bunny's way.