c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s =
   'firstname1:::surname1:::middlename1:::firstname2:::surname2:::middlename2:::firstname3:::surname3:::middlename3';
 ;;
 my $match =
 my @ra = $s =~ m{ \G ([^:]+ (?: ::: [^:]+){2}) (?: ::: | \z) }xmsg;
 ;;
 die 'no match' unless $match;
 dd \@ra;
"
[
  "firstname1:::surname1:::middlename1",
  "firstname2:::surname2:::middlename2",
  "firstname3:::surname3:::middlename3",
]