To see the certification requirements, look to Appendix B of Part 27. These are the same for any Part 27 helicopter - whether single or twin.
ICAO Annex 6 Part III also has standards for flying in IMC in Performance Class 3. These are contained in Section II Chapter 3.4, Appendix 2 and Attachment H. These SARPs are for CAT but give some indication of what might be required.
Unlike some views expressed in this and other threads, this is not a trivial issue.
Jim