You have 3 weeks! That is a lot of time. IMPROVE BOTH!!!
For Math you can easily improve if you learn the types of questions with Math Destroyer. I would do each test (takes about 2 hours for most people going through it the 1st time) yes it is painful....
But, 2nd time going through each one you learn tricks and you can do the tests timed. If you don't have the time to do this...I would suggest picking 5 tests out of Destroyer...mix it up Test 1,2,3 and 9, 10 (10 is the hardest!) and do these 5 over and over till you know them cold!!! For those who are bad at Math, doing this will best help you train your mind and help your timing for the real DAT. My real DAT was easier than math destroyer but I messed up with the calculator a lot so lost time and ended up with a 19 (Still 93rd percentile for QR). Be prepared for a horrible calculator and don't rely on it. That's my best advice for QR.
In terms of RC...what have you used to gauge a score of 16? RC is not impossible to improve like people say. If you practice reading passages in 6-7 minutes and then going through the q's and answering them the best you can....that is how you improve...PRACTICE. It is all about timing. Even if you read the passage...you will have to go back to it to find most of the answers for the q's. But, you must also answer some without going back to save time. My best advice is to read an RC passage as if you are reading it as a reporter on Television or as if it is the greatest thing in the world (I would read out loud in a whisper...helped me focus x_x). Point is: PRACTICE/FOCUS/READ/DESTROY.