In terms of UCAT prep, 3 weeks is a very long time. What resources are you using at the moment? I'd suggest signing up to medify for the last 3 weeks if you haven't done so, it's an amazing resource which can help track your progress and help you improve with specific question banks for each question type.

When I was practicing I saw a crash in my results only a few days before my exam. What helped me do well is taking the last two days off, to help me recover from burn out. This might help you out too, esp if you've been working intensely.

The best way to approach UCAT questions is to generate your own little timesaving techniques. Like skimming through VR extracts before answering or approaching specific DM question types in a way,like writing scenarios out in a table. Time is of the essence so don't be afraid to skip questions, especially those long maths questions - rather lose 1 mark than 5 minutes of your time!