Seasonal influenza poses a continuing threat to public health. The effectiveness of the influenza vaccine varies across seasons, with protection against H3N2 being notably less reliable. A primary contributor to this variability in vaccine efficacy is the frequent antigenic drift occurring in the major antigenic epitope located within the head domain of H3. In this study, we engineered stable soluble recombinant H3 trimer proteins referred to as HK68-10 and HK14-15, wherein interprotomer disulfide bonds were established to stabilize their trimerization; importantly, native antigenicity was preserved. Our design approach, devoid of conventional trimerization motifs, should be more viable and favorable for vaccine development, as it avoids off-target immune responses and reinforces structural integrity. These two H3 trimer proteins markedly augmented antibody responses towards conserved yet immuno-subdominant epitopes, thereby improving heterologous immuno-protection against H3N2 viruses. Serological experiment results demonstrate that the elicitation of serum cross-reactivity by soluble H3 vaccines depends on stem epitopes and conserved epitopes located within the head region as well. The research findings from this study are of significance for advancing future efforts to improve H3 vaccines.