设乙桥为X米;则甲桥为X+60米;丙桥为3X+800米;则
甲+乙+丙=7860;
X+60+X+3X+800=7860;
5X=7000;
X=1400米(乙桥)
甲桥为1400+60=1460米;
丙桥为:1400x3+800=5000米