
For first segment we have,
n1=2l11μT
For second segment we have,
n2=2l21μT
For third segment we have,
n3=2l31μT
For the whole string we have,
n=2l1μT
Total length of the string is,
l=l1+l2+l3
Thus, the fundamental frequency for the whole string is,
n1=n11+n21+n31