Broc Romanek reminds us that with the impending resignation of Mary Jo White as SEC Chairman, there is likely to be a period in which the SEC has only two members. (No announcement yet of who Trump plans to nominate.) But that's okay. As he points out:
Back in the 90s, President Clinton was slow to nominate new members to federal agencies and the SEC dropped down to a level of two Commissioners for a spell – Chair Levitt & Commissioner Wallman. In order to get business done, the SEC amended its rules to accommodate the Commission when it drops to such a low level. “The Rule of 2” – adopted in 1995 – is still on the books:
§200.41 Quorum of the Commission.
A quorum of the Commission shall consist of three members; provided, however, that if the number of Commissioners in office is less than three, a quorum shall consist of the number of members in office; and provided further that on any matter of business as to which the number of members in office, minus the number of members who either have disqualified themselves from consideration of such matter pursuant to §200.60 or are otherwise disqualified from such consideration, is two, two members shall constitute a quorum for purposes of such matter.
So the quorum rules are different when there are three sitting Commissioners as compared to two.
The SEC can do this because, unlike many enabling statutes creating independent agencies, the provisions of the Securities Exchange Act creating the SEC do not to speak to the issue of what constitutes a quorum. The SEC has therefore taken the position that it can set its own quorum rules.
Of course, PEOTUS Trump could make Paul Atkins' job of selecting the new SEC members a higher priority, which would help solve the problem.