Up to three medical marijuana dispensaries could be approved in the unincorporated areas of Marin County under the rules of a proposed ordinance that will be the subject of a public workshop next month. A draft of the ordinance stipulates that only one dispensary would be allowed in each of three geographic zones—the eastern corridor, the northern corridor and West Marin. The storefronts would be restricted to commercially zoned areas at least 600 feet away…