Everything everyone has mentioned here is right on point. I was never great at standardized tests but my TEAS scores were great because I made it a point to remain focused, logical, and calm during the test.
If you have 3 weeks to study, utilize those 3 weeks. Study consistently - don't cram. Make some kind of plan for yourself to make sure you don't end up cramming because it'll just make you more anxious. For instance, study for the English/grammar portions Mon and Tues each week, science on Wed and Thur, and Math on Fri-Sun. It doesn't have to be this exact model but you get the picture. The math portion was basic algebra, but it focused on concepts that I definitely had to review extensively before the exam.
Take as many practice tests as you can so that you're familiar with the way the questions will be worded. This made a huge difference for me as well, especially in maintaining my composure during the test. Since I had already seen these kinds of questions a million times in the practice tests, the only things tripping me up were the actual numbers, not the concepts, which isn't a problem as long as you take your time to do the calculations.
Stay positive and keep telling yourself you're going to do great. It really makes a difference! If you know you have test anxiety, practice some breathing exercises or make it a point to listen to a guided meditation every other night or so on youtube. It can be a 5-10 minute thing and you'd be amazed at the results! You're going to do just fine. Good luck!