What is the proper salutation for multiple females marital status unknown?

You have to use the term Ms. because you don't know their marital status. The rest depends on other aspect of the situation. If there were only two of them, you would have to say Ms. Jones and Ms. Smith. If there were several of them you might get away with writing only to one of them, presuneably the most senior as herself or as Ms Smith et al. st al is Latin for "and others". If the stuation is such that none of these are appropriate than you have to address each of them individually, Dear Ms. Smith, and Ms. Jones, and Ms. Jackson, ets.