I am sorry to say that there is not very much you can do at last minute that will improve your fitness substantially. There are no magic short cuts. It does sound like you have had some decent amount of time for exercising these past 3 weeks though so that is great. I would try to get out 1-2 days a week for extended workouts of hiking, ski touring and climbing in your local mountains during this time without exhausting yourself. If you do not have a good base of endurance then you can run a big risk of doing too much during this time so you will have to be careful to monitor your fatigue. leaving for big trip in an pre-fatigued state will not be good.