According to this having 3 axles should have nothing to do with it
261:3 Exempted Vehicles. –
I. No certificate of title need be obtained for:
(a) A vehicle owned by the United States or owned or operated by the New Hampshire National Guard;
(b) A vehicle owned by a manufacturer or dealer and held for sale, even though incidentally moved on the highway or used for purposes of testing or demonstration; or a vehicle used by a manufacturer solely for testing;
(c) A vehicle owned by a nonresident of this state and not required by law to be registered in this state;
(d) A vehicle regularly engaged in the interstate transportation of persons or property for which a currently effective certificate of title has been issued in another state;
(e) A vehicle moved solely by animal power;
(f) An implement of husbandry;
(g) Special mobile equipment;
(h) A self-propelled wheelchair or invalid tricycle;
(i) A snowmobile as defined in RSA 259:102;
(j) Trailers with gross weight of less than 3,001 pounds;
(k) Any motor vehicle whose manufacturer's model year is older than 15 years, except heavy trucks and truck-tractors whose gross vehicle weight exceeds 18,000 pounds.
II. RSA 262:1 to 5 inclusive do not apply to:
(a) A vehicle moved solely by animal power;
(b) An implement of husbandry;
(c) Special mobile equipment;
(d) A self-propelled wheelchair or invalid tricycle.
Source. RSA 269-A:2. 1967, 357:1. 1969, 488:3; 504:1