Mo. Rev. Stat. § 49.070: Quorum.

A majority of the commissioners of the county commission shall constitute a quorum to do business; a single member may adjourn from day to day, and require the attendance of those absent; when but two commissioners are sitting and they shall disagree in any matter submitted to them, the decision of the presiding commissioner shall stand as the decision of the commission; provided further, when the presiding commissioner is absent and the other two commissioners are present the commission clerk shall designate one of the commissioners present as presiding commissioner during the absence of the regular presiding commissioner, and he shall, during the absence of the regular presiding commissioner, have all of the powers of the regular presiding commissioner.