Have you been playing anything else at all? Or has it been a 3 year leave from all music?
I'd really go at it, and focus on long tones, and tone building exercises. It's like working out, because your face after all is a muscle. The more use to it by the time your audition comes around, the better you'll be. So practice everyday and never skip fundamentals. There's not really a trick or anything to it, just practicing.
Best of luck on your audition!
Marco Santos - Marcher and Performer
Guardians Drum & Bugle Corps 2015
Blue Knights Drum & Bugle Corps 2016, 20i7, 2018
Adams E1
Modified Schilke 52E2 by Justin Gorodetzky