I agree with Pete, if you just want the card then yes you can. However, if you want to be a GOOD working DM then IMO no. Sure you can get through the academics and maybe even the skills in 3 weeks but learning to DM with students takes time, effort and a good instructor. MY LDS requires you to DMC (Dive Master Canidate) for 2 complete OW classes (class, pool,OW dives- everything), 1 AOW and 1 rescue before getting your DM card. All of our skill demos must be at least a 4 with many of them a 5 (on a scale of 1 to 5), just a passing 3 will not do. This is in addition to the hours and hours spent in the pool before seeing your first student. There is no way you can do that in 3 weeks. At that point you are beginning to understand what you are doing but no where close to good at it. DMC is more about learning to deal with students, what to watch for, stopping accidents before they happen and being a good mentor than the "book learnin". Many divers think DMC is about learing diving skills, its not. You should have great dive skills before you even condsider DMC. There may be divers who can "get enough dives in" while doing their DMC and have their diving skills up to DM level but its not likely. We have all seen DMs and instructors with dive skills that are not much better than a good OW student. Its obvious that somewhere out there, there are places turning out DMs and instructors with OK (maybe) instruction skills but allowing them to do so with poor dive skills. Then there is the issue of where you intend to DM. Learning in Thailand is fine if your going to DM there but you would be unequipped to DM in North Carolina or on the west coast of the US. The diving is different. My suggestion is to reevaluate your reasons for wanting the DM card. If it's for the card, go to one of the quickie places, if you want to be a working DM, find a good instructor where you live (or want to live) and do an extended DM course. You, the instructors you work with and your students will all be glad you did.
.............Climbing down off soapbox......