Maybe (?) something like this is what you want?

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 my @triplets;
 $s =~ s{ (?= (...)) . }{ push @triplets, $1;  ''; }xmsge;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'
[download]

Update: Here's another  s/// solution. I'm not sure I like it so much: I'm always suspicious, perhaps without cause, of code embedded in a regex. In addition, the  @triplets array must be a package global (ideally local-ized) due to a bug in lexical management that wasn't fixed until Perl version 5.16 or 5.18 (I think — I don't have access to these versions and I'm too lazy to check the on-line docs).

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'ABCDEF';
 ;;
 local our @triplets;
 $s =~ s{ (?= (...) (?{ push @triplets, $^N })) . }''xmsg;
 ;;
 print qq{'$s'};
 printf qq{'$_'  } for @triplets;
"
'EF'
'ABC'  'BCD'  'CDE'  'DEF'
[download]