This answer is for valid for the US only:
47 CFR § 97.507 addresses the preparation of the telegraphy message or the question set. It does not directly address the testing session itself.
47 CFR § 97.509 addresses the testing session. Paragraph (b) states:
(b) Each administering VE must:
(1) Be accredited by the coordinating VEC;
(2) Be at least 18 years of age;
(3) Be a person who holds an amateur
operator license of the class specified
(i) Amateur Extra, Advanced or General Class in order to administer a Technician Class operator license examination;
(ii) Amateur Extra or Advanced Class in order to administer a General Class operator license examination;
(iii) Amateur Extra Class in order to administer an Amateur Extra Class operator license examination.
(4) Not be a person whose grant of an amateur station license or amateur operator
license has ever been revoked or suspended.
Since all three VEs will need to sign section 2 of form NCVEC 605 then it would make sense that all 3 VEs would hold an operator license of the class indicated it 47 CFR § 97.509.
I also found this in the [Volunteer Examiner Manual] published by the ARRL:
The three or more VEs required to oversee the separate examination area must each hold the appropriate class of FCC license as well as ARRL VE accreditation.
So again, it seems that if you want to administer a test, you would need 3 VEs that hold an operator license of the class indicated it 47 CFR § 97.509.
: http://www.arrl.org/files/file/VEs/VE%20Manual%20Web%20FINAL%202013.pdf, page 46.