To do not involve some heavy instruments (like sed or awk):

paste -d',' <(cut -d',' -f-2 file) column.file <(cut -d',' -f3- file)

To do not involve some heavy instruments (like sed or awk):

paste -d',' <(cut -d',' -f-2 file) column.file <(cut -d',' -f3- file)

This assumes that the column you want to add is saved in the file column.file.