Bone mass is determined by coordinated acts of osteoblasts and osteoclasts, which control bone formation and resorption, respectively. Osteoclasts are multinucleated, macrophage/monocyte lineage cells from bone marrow. The Dok-family adaptors Dok-1, Dok-2 and Dok-3 are expressed in the macrophage/monocyte lineage and negatively regulate many signaling pathways, implying roles in osteoclastogenesis. Indeed, mice lacking Dok-1 and Dok-2, the closest homologues with redundant functions, develop osteopenia with increased osteoclast counts compared to the wild-type controls. Here, we demonstrate that Dok-3 knockout (KO) mice also develop osteopenia. However, Dok-3 KO, but not Dok-1/-2 double-KO (DKO), mice develop larger osteoclasts within the normal cell-count range, suggesting a distinctive role for Dok-3. Indeed, Dok-3 KO, but not Dok-1/-2 DKO, bone marrow-derived cells (BMDCs) generated larger osteoclasts with more nuclei due to augmented cell-to-cell fusion in vitro. In addition, while Dok-1/-2 DKO BMDCs generated more osteoclasts, Dok-1/-2/-3 triple-KO (TKO) BMDCs generated osteoclasts increased in both number and size. Furthermore, Dok-1/-2/-3 TKO mice showed the combined effects of Dok-3 and Dok-1/-2 deficiency: severe osteopenia with more and larger osteoclasts. Together, our findings demonstrate that Dok-3 and Dok-1/-2 play distinctive but cooperative roles in osteoclastogenesis and protect mice from osteopenia, providing physiological and pathophysiological insight into bone homeostasis.