I think it would be possibly depending upon how well you did in your science courses, how strong your math background is, and how quickly you understand the PAT.
Assuming you are strong in science / math and that you have innate PAT skills, I think 3 weeks of the following would have gotten me about as much practice / studying as I achieved in the 6 weeks I actually studied for the DAT. You will probably need to study a solid 8 - 10 hours each day for all 3 weeks.
1. Read Cliff's 4 times and supplement on the first pass with Campbell's. I also advise taking notes on the first pass, just to solidify your understanding of the material. This seems like a lot, but you can do it in a week if you really make yourself do it every day. It was on the 4th pass that I solidified and fully integrated my biological understanding, that's why I say 4 passes (I scored a 28 on bio and there were a few questions I wouldn't have been able to answer without the 4th reading). The 1st pass is a review, the 2nd pass is to find your weaknesses, the 3rd pass is to refine your understanding, the 4th pass will integrate everything. Also, save the memorization stuff (phylums, hormones, etc.) until the last few days before the exam.
2. Get through DAT Destroyer GC, OC, and Bio 3x (2 days for each 1st pass which is 150 questions for ochem / gen and 250 for bio, then 1 day each for the subsequent passes). That will be a total of 12 days.... But, it will be well worth it. Again, difficult, but possible. The first time through will take about as long as the next two times combined if you are diligent on the first pass. My last pass through Destroyer Bio I did 400 of the 520 problems in 4 hours. Your first time through will be frustrating, and it will make you think that your timeline is impossible. Just work through the pain, and you'll see that your 2nd and 3rd passes are MUCH easier.
3. Watch all of Chad's videos once at normal speed while taking notes, and then again a week later at double speed. It took me 2 days to watch each chem section with quizzes, but I could have done each in a day. Take the quizzes after each video, and then do quizzes without watching to find your weak spots. If you suck on a quiz, then watch the corresponding video.
4. Do DAT Destroyer's math section. I am assuming you are strong in math, otherwise 3 weeks is unlikely.
5. Use Bootcamp + Youtube to study for PAT. If you are really struggling with PAT then 3 weeks may also be unlikely. If you find the PAT reasonable, then 3 weeks is doable.
12-14 days with DAT Destroyer and Chad's, 7-9 days with Cliff's and Campbell's. Hit the PAT intermittently to complete 10 full practice exams (that's only 10 hours in total).
