I meant all 3letter words that occur more than once.

In that case:

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;
}
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