Please better define what you mean with "three letter-doubles". The two monks that responded previously understood something different, and I understood yet a third possibility.
use strict;
use warnings;
my $term = 'Dit is het eerste het is niet het laatste Dit';
my @tlw = $term =~ /\b(\w{3})\b/g;
# Now you have all the three-letter words, so count them
my %seen = ();
$seen{$_}++ for @tlw;
for my $k (keys %seen) {
print "$k occurs $seen{$k} times\n" if $seen{$k} > 1;
}