String A is 30cm longer than string B? If so here is the solution
Let string C be of length= x cm
length of string B=x+60 cm
length of string A=x+60+30=x+90 cm
total length=3m=300cm
x+x+60+x+90=300
3x+150=300
3x=150
x=150/3=50 cm
Length of string C=50 cm