>perl -wMstrict -le
"use List::Util qw(shuffle);
 my @set = qw(a b c d e f);
 my $len = 12;
 my $needed = shift;
 my @out;
 while (@out < $needed) {
   my $str = join '', shuffle((@set) x $len);
   push @out, grep !m{ (.)\1\1 }xms, $str =~ m{ .{$len} }xmsg;
   }
 @out = @out[-$needed .. -1];
 m{ (.)\1\1 }xms and die qq{3+ same: '$_'} for @out;
 printf qq{%3d: '$out[$_]' \n}, 1+$_ for 0 .. $#out;
" 5
  1: 'becaecdbbedc'
  2: 'afccbecefdca'
  3: 'fbffecacfeaa'
  4: 'ddbacddfafca'
  5: 'fbebdabadedc'