Current treatment options for patients with advanced colorectal cancers (CRC) include anti-EGFR/HER1 therapy with the blocking antibody cetuximab. Although a subset of patients with KRAS wild-type disease initially respond to the treatment, resistance develops in almost all cases. Relapse has been associated with the production of the ligand heregulin (HRG) and/or compensatory signaling involving the receptor tyrosine kinases HER2 and HER3. Here we provide evidence that triple HER receptor blockade based on a newly developed bispecific EGFRxHER3-targeting antibody (scDb-Fc) together with the HER2 blocking antibody trastuzumab effectively inhibited HRG-induced HER receptor phosphorylation, downstream signaling, proliferation and stem cell expansion of DiFi and LIM1215 CRC cells. Comparative analyses revealed that the biological activity of scDb-Fc plus trastuzumab was sometimes even superior to that of the combination of the parental antibodies, with PI3K/Akt pathway inhibition correlating with improved therapeutic response and apoptosis induction as seen by single cell analysis. Importantly, growth suppression by triple HER targeting was recapitulated in primary KRAS wild-type patient-derived organoid (PDO) cultures exposed to HRG. Collectively, our results provide strong support for a pan-HER receptor blocking approach to combat anti-EGFR therapy resistance of KRAS wild-type CRC tumors mediated by the upregulation of HRG and/or HER2/HER3 signaling.