You will definitely have to keep the chickens out of the worm bins. In fact, I suggest you keep all three separate. They do work well as a homesteading combination though.
Lamb's Quarters makes a good rabbit feed when it is young, but they don't like it much once it matures. If you have an area of rough "lawn" blessed with weeds, you have a great place for gathering greens, as long as you don't use pesticides or herbicides on it and as long as it is not contaminated by dog or cat feces. Be sure to take a look at the Natural Feeding forum - in particular the Safe Plants sticky - for ideas about what weeds to look for. The leaves and twigs of a number of trees are also very useful: apple, pear, elm, willow, poplar and mulberry, to name a few. Raspberry, rose and blackberry bushes are also great. It is fascinating the variety of safe and healthy feed plants that are available for feeding rabbits.