Forecasting in Health Care

#1
Hello,
I'm in charge of forecasting heath care costs for an insurance company. One large driver of costs is Outpatient Surgeries (OPS). We prepare estimates for how many OPS our members incur (the OPS "utilization") so we can calculate how much premium to charge. Sometimes, OPS utilization exceeds our forecast and we lose money. The challenge is to forecast the utilization accurately so we know how much premium to charge.

Here is an example of OPS utilization by month:
Jan/Feb/Mar/Apr/May/.....

Year 1 757/770/862/778/732/792/683/828/793/772/820/889
Year 2 802/787/915/783/874/825/754/814/746/812/771/801
Year 3 802/758/856

What test would I use to determine whether the first three months of Year 3 are significantly different from the past 2 years?

Ideally, could someone suggest a good book that might deal with forecast vs. actual variability, trends, control limits, etc?