She should come off the nest once a day to eat, drink, and do her business. However, some hens don't. So you may have to put her off the nest every day. I usually pull them off once a day to make sure they eat, whether they have enough sense to come off the nest on their own or not. But truthfully, I love to interfere in the brooding process. It's so much fun!
I like to give my broody girls moistened chick food. I put it in a plastic container, add water to it, and they really go crazy for it. They seem to eat much more then if I just let them eat whatever is in the feeders for the flock. Seems that they lose less weight that way.
As far as whether or not to separate her. Well, it depends. If you only have the 3 hens, and the other 2 spend most of their time in the garden, everything may be fine. I keep my broodies in separate broody houses, but the broody houses are in the chicken pen with the flock. My broody girls are tough, and the flock isn't very aggressive in general, so I haven't had any problem with that setup. You will have to evaluate the personalities of your particular hens. Normally a broody momma is plenty aggressive enough to protect her babies from the other hens.
Good luck with your hatch!