Appending = in particular column in csv file

Hi,

I have a requirement to append = in particular row in csv file. Data in csv is as follow:

row1,a,a,a
row2,b,b,b
row3,c,c,c
row4,d,d,d

csv should be modified at row3 and no. of columns are not fixed but rows are. output should be as:

row1,a,a,a
row2,b,b,b
row3,=c,=c,=c
row4,d,d,d

= should be appended after comma.

Please help.

sed '3''s/,/,=/g' file.csv

awk 'NR==3{gsub(",",",=",$0)}1' file.csv