How much weight should women lift, MAX if they are looking only to tone and get lean muscles?
That all depends on the individual and the exercise. It's impossible to paint all women (with body types, experience and goals) with the same MAX.
I do legs 3 days a week because I love it and I enjoy having lean muscle in my legs
You can accomplish having lean, strong legs without doing them 3 days a week. Frankly, you're over training them. Either your 3 workouts aren't enough of a challenge to promote change and progress or you aren't allowing your legs and central nervous system enough time to recover, repair and improve. My guess would be the latter. You will find even body builders do not work out one particular body part as frequently. Rest is as important to your muscles as is the workout itself, if not more so.
Also, has anyone ever used the "ab-coaster" in the gym?
Why would you need an Ab Coaster? Especially in a gym? The Ab Coaster does nothing, I repeat, absolutely nothing different than any other combination of ab workouts available without a machine. What allows your abs to show in your stomach is the absence of fat, which of course begins with diet. Anyone on this forum, or pretty much any professional trainer would agree. What allows your abs to "pop" beyond the average physique is weight training/resistance training those muscles. Weighted crunches, weighted Russian twists or bicycle crunches. All the Ab Coaster does, and ANY exercise does, is make you move... movement of your muscles causes calories/energy to be burned/used, which in turn (with diet) burns fat.
I would look into your diet, determine your daily and weekly caloric intake and create a deficit from what you should be taking in (there's a good calculator available on this site for you to determine how many calories you should take in). I would also recommend looking into an overall workout, which incorporates squats and deadlifts (if you REALLY want nice lookin' legs).
Hope this helps.