I'm sorry, but I have to ask: does this mean that 45% of your customers are currently dissatisfied? Yikes.
Whether or not it is "realistic," I think a three-year plan is dragging out the turnaround that is drastically needed. If it were up to me, I would prioritize the problems of that 45%, by potential revenue and industry influence, and get right on the case. Forget about 3 years! You might not be around that long!
My gut feeling is that when a company has 45% of its customers dissatisified with their service, it says a couple of things. This kind of service rating is not the doing of the front line service providers. Responsibility for this has to lie on the heads of senior management. Their business strategy is obviously not customer-focused, and until that occurs, all the three year plans and best-in-class metrics are not going to mean a thing. I think simply by virtue of the fact that your client is more concerned about overall benchmark metrics rather than fixing specific customer issues one by one indicates that your client still does not "get it." Customer service is not about metrics, it is about customers. Period.
Good luck with this.