Mechanical behaviours of three-tower suspension bridges from perspective of influence line help insight into mechanical behaviour nature, and grasp the differences between two-tower and three-tower suspension bridges in Static and dynamic behaviours. Based on the Taizhou Yangtze River Bridge, three dimensional space finite element models of two-tower, three-tower with floating system and three-tower with elastic cables between mid-tower and the girder were constructed. Through analysis of influence lines characteristics of different key effects and effect envelop under vehicular load, main conclusions are got as follows: Some factors such as deflection-to-span ratio of girder, anti-slipping safety factor between the main cable and saddle of the mid-tower and force in the mid-tower, horizontal displacement at the end of girder, which are not important in two-tower design, become controlling ones in three-tower suspension bridges and effects under vehicular load is the most important. When live load acts on all or part of one main span, the four control factors reach the worst case, but in very low probability. Code restriction on deflection-to-span ratio of girder can be flexible to relax restriction to 1/220, and riding comfort is still guaranteed. Stresses of mid-tower under vehicular load plus dead load are in a state of compressive stress in the majority of operating conditions and fatigue problem is not obvious.