Unless the organisation's constitution specifies the form that voting must take, the Chairman would normally be free to determine the procedure (subject to the constraint that any such procedure must be seen to be fair).
The usual format (as adopted, for example, by parish councils with more than one seat to fill) would be to simply list all of the candidates (in this case three) and to require voters to mark an 'X' against a number of candidates not exceeding the number of vacant posts (in this case two).
So you should list all three names on the voting forms and tell those voting that they can mark either one or two crosses on the form. (The option to mark only one cross is important because someone might, for example, really want A to be elected but have no knowledge of either B's or C's qualities).