In this paper, frequencies of low order vertical modals for three-tower self-anchored suspension bridge (TSSB) are studied. The first three-tower self-anchored suspension bridge is taken as prototype in the paper. The finite element model of the bridge is established. And dynamic characteristics are analyzed. Based on Rayleigh method, frequency formulas of 1st asymmetric vertical vibration (AVV) and symmetric vertical vibration (SVV) are deduced considering contribution of towers stiffness. The accuracy of deduced formulas was validated by results of numerical analysis and modal test. Stiffness characteristic difference between TSSB and multi-tower earth-anchored suspension bridge (MESB) is researched based on frequency formulas. The significant difference is gravity stiffness component in frequency formulas. At last, influence law of tower stiffness on vertical frequencies was studied based on validated formulas. The results indicated that: 1) middle tower stiffness and side tower stiffness play important role in frequency of 1st AVV and SVV respectively. Frequencies are enlarging as tower stiffness increasing; 2) when tower stiffness is low, deduced frequency formulas are the same as formulas neglecting influence of tower stiffness in reference paper; 3) based on deduced frequency formulas, expressions of critical tower stiffness for simplified formulas are proposed.