N.H. Rev. Stat. Ann. § 407-B:6

No experience rating shall be applied to an individual group until after 3 years of operation and at that time shall be based on the experience of the preceding 3 calendar years. Thereafter, any experience rating shall be made annually using the last 3 calendar years. In case of merger of 2 or more companies, the 3-year minimum will apply to the oldest company in the newly-formed entity, not the new entity formed by the merger itself.