Phase 1: -To determine the maximum tolerate dose (MTD)/maximum administered dose (MAD) and the Phase 2b dose of the ibrutinib, pomalidomide and dexamethasone combination. - To determine the safety and tolerability of ibrutinib in combination with pomalidomide and dexamethasone in subjects with relapsed/refractory Multiple Myeloma (MM). Phase 2b: - To evaluate the effect of ibrutinib in combination with pomalidomide and dexamethasone compared to placebo in combination with pomalidomide and dexamethasone on progression-free survival (PFS), as assessed by independent review committee (IRC), in subjects with relapsed/refractory MM.