Snmp V3는 달라진 것보다는 추가된것이 라고 보는게 맞는것 같고요..
보안이란 부분은 일부분일것 같습니다. 간단히 예를 들자면 SSL/TLS (HTTPS)을 통한 고차원적 보안 정도 같이 보면 되겠죠.. 아래 내용을 참조하시기 바랍니다. 저도 개발자가 아니라서 찾아보는 수준밖에는.. ㅋㅋㅋㅋ

SNMP V3

SNMPv2 : RFC1905, RFC1907
SNMPv3 : RFC2571, RFC2572, RFC2264

SNMP v3, SSL/TLS (HTTPS)을 통한 고차원적 보안


http://www.ietf.org/html.charters/snmpv3-charter.html
http://www.simple-times.org/

아래는 여러 SNMP관련 RFC문서중에 Security 부분을 발췌한 내용입니다.
http://www.ietf.org/rfc/rfc2264.txt

User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)

1.3.  Security Services

   The security services necessary to support the goals of this SNMP
   Security Model are as follows:

   - Data Integrity
     is the provision of the property that data has not been altered or
     destroyed in an unauthorized manner, nor have data sequences been
     altered to an extent greater than can occur non-maliciously.

   - Data Origin Authentication
     is the provision of the property that the claimed identity of the
     user on whose behalf received data was originated is corroborated.

   - Data Confidentiality
     is the provision of the property that information is not made
     available or disclosed to unauthorized individuals, entities, or
     processes.

   - Message timeliness and limited replay protection
     is the provision of the property that a message whose generation
     time is outside of a specified time window is not accepted.  Note
     that message reordering is not dealt with and can occur in normal
     conditions too.

   For the protocols specified in this memo, it is not possible to
   assure the specific originator of a received SNMP message; rather, it
   is the user on whose behalf the message was originated that is
   authenticated.

   For these protocols, it not possible to obtain data integrity without
   data origin authentication, nor is it possible to obtain data origin
   authentication without data integrity.  Further, there is no
   provision for data confidentiality without both data integrity and
   data origin authentication.

   The security protocols used in this memo are considered acceptably
   secure at the time of writing.  However, the procedures allow for new
   authentication and privacy methods to be specified at a future time
   if the need arises.


좋은하루 되세요.. ^^