The problem may have something to do with the structure of your data. Does the following example work for you in displaying all 3 values for the legend?

use GD::Graph::hbars;
use strict;
use warnings;
my @data = (
            ['Jan', 'Feb', 'Mar', 'Apr'],
            [5, 7, 9, 6],
            [2, 2, 3, 1],
            [4, 4, 6, 3],
           );
my @legend = qw(maximum minimum average);

my $graph = GD::Graph::hbars->new(600, 500);
$graph->set (
             x_label => 'Month',
             y_label => 'Value',
             title => 'GD::Graph::hbars example',
             bar_spacing => 6,
             show_values => 1,
             legend_placement => 'BL',
             legend_spacing => 5,
             show_values => 1,
             legend_marker_height => 12,
             dclrs => [ qw(orange cyan lred) ],
             ) or warn $graph->error;

$graph->set_legend(@legend);
my $gd = $graph->plot(\@data) or warn $graph->error;

open(IMG, ">graph.png") or die $!;
binmode IMG;
print IMG $gd->png;
close IMG;
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