An isolated heterotrimer protein that binds a first and second antigen of interest in monovalent fashion, consisting of: (a) a first polypeptide chain comprising a first variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit), a second variable domain, and a human Fc domain capable of FcRn binding interposed between the first and second variable domains, wherein the V-(CH1/CK) unit is fused to the human Fc domain via a hinge region; and (b) a second polypeptide chain comprising a first variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit) selected to be complementary to the CH1 or CK constant region of the first polypeptide chain such that the first and second polypeptides form a CH1-CK heterodimer in which the first variable domain of the first polypeptide chain and the first variable domain of the second polypeptide form an antigen binding domain that binds the first antigen of interest, wherein the V-(CH1/CK) unit is fused to a human Fc domain via a hinge region, and (c) a third polypeptide chain comprising a second variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit) selected to be complementary to the CH1 or CK constant region of the first polypeptide chain such that the first and third polypeptides form a CH1-CK heterodimer in which the second variable domain of the first polypeptide chain and the second variable domain of the third polypeptide form an antigen binding domain that binds the second antigen of interest, wherein the hetero-multimeric polypeptide is a trimer with a dimeric Fc domain, having the domain arrangement: wherein each V-V pairing occurs between a light chain variable domain and a heavy chain variable domain, and each constant region pairing occurs between a CH1 and a CK. (a) a first polypeptide chain comprising a first variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit), a second variable domain, and a human Fc domain capable of FcRn binding interposed between the first and second variable domains, wherein the V-(CH1/CK) unit is fused to the human Fc domain via a hinge region; and (b) a second polypeptide chain comprising a first variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit) selected to be complementary to the CH1 or CK constant region of the first polypeptide chain such that the first and second polypeptides form a CH1-CK heterodimer in which the first variable domain of the first polypeptide chain and the first variable domain of the second polypeptide form an antigen binding domain that binds the first antigen of interest, wherein the V-(CH1/CK) unit is fused to a human Fc domain via a hinge region, and (c) a third polypeptide chain comprising a second variable domain V fused to a CH1 or CK constant region (a V-(CH1/CK) unit) selected to be complementary to the CH1 or CK constant region of the first polypeptide chain such that the first and third polypeptides form a CH1-CK heterodimer in which the second variable domain of the first polypeptide chain and the second variable domain of the third polypeptide form an antigen binding domain that binds the second antigen of interest, wherein the hetero-multimeric polypeptide is a trimer with a dimeric Fc domain, having the domain arrangement: wherein each V-V pairing occurs between a light chain variable domain and a heavy chain variable domain, and each constant region pairing occurs between a CH1 and a CK.