How to append a line in a file?

How to append a line in a file containing three columns, when ever third column value changes i have append a line from a different file containing data of three columns. For example
when there is a change in third column of first file , i have to append a line from an another file's first line containing three columns, similarly when there is another change in third column of first file , i have to append a line from an another file's second line containing three columns. Till there is no change in the third column of the first file

Please check the files attached


first.txt
second.txt