/(?<=222,)(\d{3})/g

##</code><code>##

$ perl -E 'say for "222,222,123" =~ /(?<=222,)(\d{3})/g'
222
123
$ perl -E 'say for "222,345,222,678,222,222,543,111" =~ /(?<=222,)(\d{3})/g'
345
678
222
543
$ perl -E 'say for "123,222,456,222,222,111,222,999" =~ /(?<=222,)(\d{3})/g'
456
222
111
999