This regular expression will match 6 groups of 0+ non-comma characters for each line:
^([^,]*),([^,]*),([^,]*),([^,]*),([^,]*),([^,]*)$
You can then replace with your captured groups like so:
\1,\4,\3
Side note:
Someone correct me if I'm wrong. I've been trying to reduce this down to just one repeated capturing group for legibility, but I can't seem to make it work since it only sees this as one capture group:
^([^,]*,?){6}$