A:
Unless your bylaws or the board's own rules state otherwise (and they often do), a board may conduct business only at regular or properly called meetings, or at an adjourned meeting.
(RONR p. 476.)
The bylaws example given in RONR p. 580, says "... Special meetings of the Board may be called by the President and shall be called upon the written request of three members of the Board."
Bottom line: Check your bylaws and board rules.