A multispecific antibody comprising at least three antigen binding sites, wherein two antigen binding sites are formed by a first Fab fragment and a second Fab fragment, wherein a) a third antigen binding site is formed by a variable heavy chain domain (VH 3) and a variable light chain domain (VL 3), wherein - the N-terminus of the VH3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the first Fab fragment via a first peptide connector, and - the N-terminus of the VL3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the second Fab fragment via a second peptide connector, b) the multispecific antibody comprises two constant heavy chain domains 3, which are altered to promote heterodimerization by i) generation of a protuberance in one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a larger side chain volume than the original amino acid residue, and generation of a cavity in the other one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a smaller side chain volume than the original amino acid residue, such that the protuberance generated in one of the CH3 domains is positionable in the cavity generated in the other one of the CH3 domains; or substituting at least one original amino acid residue in one of the CH3 domains by a positively charged amino acid, and substituting at least one original amino acid residue in the other one of the CH3 domains by a negatively charged amino acid; ii) introduction of at least one cysteine residue in each CH3 domain such that a disulfide bond is formed between the CH3 domains, or iii) both modifications of i) and ii); c) the C-terminus of the VH 3 domain of the third antigen binding site is connected to one of the CH3 domains, and the C-terminus of the VL 3 domain of the third antigen binding site is connected to the other one of the CH3 domains, and d) the multispecific antibody is devoid of constant heavy chain domains 2. a) a third antigen binding site is formed by a variable heavy chain domain (VH 3) and a variable light chain domain (VL 3), wherein - the N-terminus of the VH3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the first Fab fragment via a first peptide connector, and - the N-terminus of the VL3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the second Fab fragment via a second peptide connector, - the N-terminus of the VH3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the first Fab fragment via a first peptide connector, and - the N-terminus of the VL3 domain is connected to the C-terminus of the constant heavy chain domain or the constant light chain domain of the second Fab fragment via a second peptide connector, b) the multispecific antibody comprises two constant heavy chain domains 3, which are altered to promote heterodimerization by i) generation of a protuberance in one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a larger side chain volume than the original amino acid residue, and generation of a cavity in the other one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a smaller side chain volume than the original amino acid residue, such that the protuberance generated in one of the CH3 domains is positionable in the cavity generated in the other one of the CH3 domains; or substituting at least one original amino acid residue in one of the CH3 domains by a positively charged amino acid, and substituting at least one original amino acid residue in the other one of the CH3 domains by a negatively charged amino acid; ii) introduction of at least one cysteine residue in each CH3 domain such that a disulfide bond is formed between the CH3 domains, or iii) both modifications of i) and ii); i) generation of a protuberance in one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a larger side chain volume than the original amino acid residue, and generation of a cavity in the other one of the CH3 domains by substituting at least one original amino acid residue by an amino acid residue having a smaller side chain volume than the original amino acid residue, such that the protuberance generated in one of the CH3 domains is positionable in the cavity generated in the other one of the CH3 domains; or substituting at least one original amino acid residue in one of the CH3 domains by a positively charged amino acid, and substituting at least one original amino acid residue in the other one of the CH3 domains by a negatively charged amino acid; ii) introduction of at least one cysteine residue in each CH3 domain such that a disulfide bond is formed between the CH3 domains, or iii) both modifications of i) and ii); c) the C-terminus of the VH 3 domain of the third antigen binding site is connected to one of the CH3 domains, and the C-terminus of the VL 3 domain of the third antigen binding site is connected to the other one of the CH3 domains, and d) the multispecific antibody is devoid of constant heavy chain domains 2.