Yes, this looks like homework.

From your example input/output, it appears that you are only interested in whether a particular key occurs in common between the hashes (the value of those keys doesn't matter). Consider:

#!/usr/bin/perl -w
use strict;

my %hash1 = ( A => 1, B => 2, C => 3, D => 4, ); 
my %hash2 = ( D=>1, E =>2, F => 3, G => 4, H => 5, I => 6, J=>7 ); 
my %hash3 = ( J=>1, K =>2, L => 3, M => 4, N => 5, O => 6, P=>7, 
              Q =>8, R =>9, S =>10, T => 11, U =>12, D=>13, );

foreach my $key ( keys(%hash1), keys(%hash2), keys(%hash3) )
{
   print "$key";
}
print "\n";
#prints:ADCBHFJDIGESOJTNPKQMDRLU
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One property of a hash is that the key is "unique" - it can only occur once in a particular hash.
Hashes are good for "counting number of occurrences of things".
In the above, since 'D' occurs 3 times, it is possible to conclude that they key 'D' appears in common with all 3 hashes.
I will leave it to you as to how to use this example in your homework.