Gaining muscle whilst you are still losing weight shouldn't be a problem -- just so long as you continue doing your cardio-type workouts to keep the fat coming off. After all, once the muscle is built up, that will need fuelling, so should stip out a few more calories.
Just remember that muscle is heavier than fat. If you find your weight going up, but your leanness improving (with no extra fat), don't fret!
As for getting rid of all that weight so quickly, if you're not starving yourself, you should be alright. However, 3lbs/week is quite a lot (I fear a reasonable proportion of that will be water, too), so you might want to monitor your weight closely, otherwise it may blow back up... Remember, dieting and exercising isn't just a quick fix -- it requires a permanent change in lifestyle to keep up.
