Physical therapy is essentially applied kinesiology. PT schools are generally more worried about your grades and GPAs than your actual major itself (as long as it's health science-oriented), so pick your major based on what interests you the most rather than what you think is more marketable. The more interested you are in your major, the more you'll enjoy your classes, and the better your grades will be.
If you know for sure that physical therapy is what you want to do for the rest of your life, then I would definitely recommend looking into 3+3 programs. It will save you both time and money. However, if there is any doubt in your mind about your future career, then I would avoid this option.
As far as the application process goes, it is definitely harder than college applications. The Physical Therapy Centralized Application System (PTCAS) definitely makes life a little bit easier, but so far only about 50% of all PT schools use it. As I mentioned before, the most important thing is your grades, but you'll want to focus on other areas as well in order to make you more competitive.
-Get a lot of volunteer/experience hours at a variety of clinics in different settings.
-Develop a good relationship with a college professor (when you get there) so they can write you a good letter of recommendation. I did this by doing research for one.
-Develop a good relationship with 2 physical therapists for the same reason as mentioned above. The best way to do this is work hard and go the extra mile when you're volunteering/working for them.
-Get involved in extracurricular activities. If you're in an organization, try to attain a leadership role within it. I was in a fraternity and managed to become president.
-Work on your interpersonal skills. Identify your weaknesses when dealing with other people and make a conscious effort to try to turn those weaknesses into strengths.
-Finally, stay determined and motivated. Becoming a PT isn't easy and you may get discouraged at times. You have to have a genuine love for what you are doing or else it will be very difficult.
Good luck!