I purchased a small coop (about 4 feet wide, 2 1/2 feet deep, 4 feet high with one foot legs) that has a removeable pan that I line with newspaper. There's a wire floor, so the poop just drops through to the paper. This is very convenient, as they just sleep here and lay their eggs in the nesting box in the back. The only litter I use are pine shavings in their laying box. Others use shredded newspaper, hay, whatever, just make sure it's edible (just in case, they will give it a taste), non-toxic and relatively low in dust. Though they don't typically mess in their laying box, they do hit it now and then while roosting, and once a week I just dump the whole thing in the compost.
They'll do the majority of pooping on the ground or wherever, like you said. I have friends that put down hay, and the girls seem to like it, more to scratch around and find bugs. I agree with the previous post...when it gets wet, it can mildew and cause big problems and a big mess for you to clean up. If they have access to grass and dirt, you probably won't need litter, especially with just 2 or 3 birds. At least, I have not found a need for it!