I see two top priorities in the first 100 days, and they both involve assessment, and they should be done simultaneously.
1. Assess the skills, competency, and execution of your customer service staff. Determine who does what well, who needs addition training / motivation in what areas, and who is just not going to deliver what you require out of that position (then begin plans for replacement).
2. Assess the satisfaction of your top tier and second tier customers (which is not to say ignore the lower tier customers - you'll get to them after the first 100 days!). Look for trends in both satisfaction and dissatisfaction. Emphasize and reward the things that bring satisfaction, and create training and coaching around those things that bring dissatisfaction.
That ought to keep you busy for those first 3 months.
If I can be of any assistance in any of these tasks, feel free to contact me.
Regards, and best of luck,