Additional Study Information: This is a research study. We invite you to participate in this research study because you have been diagnosed with Multiple Myeloma (MM). Your previous treatment(s) has/have not been successful and your cancer is progressing. The purpose of this research study is to see if selinexor (also known as KPT-330) has any effects on your cancer. Selinexor is considered to be an investigational drug, which means it has not been approved by the U.S. Food and Drug Administration (FDA), Health Canada, the European Medicines Agency (EMA) or any other regulatory agency. The study drug (selinexor) works by keeping these tumor suppressor proteins within the cell and thus causing the cancer cells to die or stop growing. Selinexor has previously been tested in humans to identify a safe dose for providing to subject(s)/participant(s) with cancer. At this time, it is not known if selinexor will have an effect on your cancer. Selinexor is also being tested in other clinical studies in subject(s)/participant(s) with other advanced cancers. This study will look at the effects of different dose schedules of selinexor in two treatment combinations on your cancer and on your body, including any side effects that you may experience. This is an open-label study, which means that both you and your study doctor will know how much selinexor and other medications you are taking. You, and all other subject(s)/participant(s) in this study, will take the study drug selinexor, plus dexamethasone and either pomalidomide, bortezomib, or lenalidomide. The treatment period consists of blocks of time called cycles. The number of treatment cycles you receive will depend on how well you are doing on the treatment and how well your cancer responds to it.