If I had to limit myself to just three things to focus on, here's what I would attempt to do:
Stay on top of industry trends - know what's happening in your industry; know what "best-in-class" companies do; benchmark if possible; know what your competitors are doing.
Challenge the status quo; challenge yourself (and others) to continuously strive for excellence. Hold yourself accountable for creating value. Scorecard your performance at the end of each week - how did you improve service / reduce cost / improve processes, etc. Remember, accumulated incremental changes can become transformational.
Develop as many relationships as you can - internal / external and cross-functional. Avoid becoming siloed and making siloed decisions. Understand the upstream and downstream impact of the decisions that you make. Strive to build and maintain high-functioning, healthy working relationships.