Three materials (agar, konjac glucomannan (KGM) and κ-carrageenan) were used to prepare ternary systems, i.e., sol-gels and their dried composites conditioned at varied relative humidity (RH) (33%, 54% and 75%). Combined methods, e.g., scanning electron microscopy, small-angle X-ray scattering, infrared spectroscopy (IR) and X-ray diffraction (XRD), were used to disclose how κ-carrageenan addition tailors the features of agar/KGM/κ-carrageenan ternary system. As affirmed by IR and XRD, the ternary systems with κ-carrageenan below 25% (agar/KGM/carrageenan, 50:25:25, m/m) displayed proper component interactions, which increased the sol-gel transition temperature and the hardness of obtained gels. For instance, the ternary composites could show hardness about 3 to 4 times higher than that for binary counterpart. These gels were dehydrated to acquire ternary composites. Compared to agar/KGM composite, the ternary composites showed fewer crystallites and nanoscale orders, and newly-formed nanoscale structures from chain assembly. Such multi-scale structures, for composites with κ-carrageenan below 25%, showed weaker changes with RH, as revealed by especially morphologic and crystalline features. Consequently, the ternary composites with less κ-carrageenan (below 25%) exhibited stabilized elongation at break and hydrophilicity at different RHs. This hints to us that agar/KGM/κ-carrageenan composite systems can display series applications with improved features, e.g., increased sol-gel transition point.