I do the tae bo flex video a lot.
You need to take it slow to start with when you're unfit. It can damage your muscles if you start when you aren't ready, i was reading an article on it last night (search in google) and this highlighted the dangers of tae bo for unfit people.
I find that it's a great workout, but I think if you've only been working out for 3 weeks, you should leave it for another 3 weeks OR take it VERY slowly. A good indicator is that if you can do a whole aerobics DVD (like hotpants) vigorously then you can start Billy Blanks properly.