Honestly, you won't feel that much more prepared at the end of the 1 or 2 weeks that you push your exam back to. I would say to use your 3 weeks wisely, since that is plenty of time. I studied for approximately a month with 5-6 hours a day on average and felt that was plenty of time (but of course, I felt like I was running out of time/wanted more time to study).
I would do all of the bootcamp tests after you have a solid foundation with content review, mark down the questions you get wrong and make notes on them. I would also do Destroyer at the same time and also mark each question you get wrong (do it one time through without writing in the book, and then the second time you can write in the book).
Do the 2007 ADA DAT (free) around 2 weeks before your exam. Then do the 2009 ADA exam (online or hard copy) a week or at least 3 days before your exam (so you have time after to calm down and recollect yourself).
Make sure that you do a lot of content review and use Destroyer mostly as a study tool (it is harder than the actual, so don't be discouraged). Do not delay QR and PAT as well. 2 weeks worked for me for both these sections, but I had a good foundation in QR but did not do stellar on the PAT. RC should also be practiced, so try to do all the bootcamp tests within a week (one every other day) to get a good feel for timing/what strategies work best for you.
Set a deadline for yourself, and stay motivated. If you absolutely feel like you won't be ready in 3 weeks, then by all means push the exam back. However, 3 weeks is plenty of time. It's all about being efficient and tackling your weaknesses. Each second is an opportunity to learn something new. Good luck!
