i suggest using a bit vector for storage (see vec.) use one bit per number to store whether it's triangular or not. i'll leave it as an excersize to figure out the details of my implementation.

#!/usr/bin/perl -w
use strict;
$|++;


my $number = $ARGV[0] || die("Usage: trivec.pl number$/");
my $vecTri;

$vecTri = createTriVec( $number );
print $/, findTriCombo( $vecTri, $number );

sub createTriVec
{
    my $upper = $_[0] || 99;
    my $vec = "\0";
    my $guess = 0;
    for( my $num = 1; $guess+$num <= $upper; $num++ )
    {
        $guess+=$num;
        vec( $vec, $guess, 1 ) = 1;
    }
    return $vec;
}

sub findTriCombo
{
    my $vec = $_[0] || return undef;
    my $num = $_[1] || return undef;
    my @tris;
    my $sum = 0;
    push(@tris, $num, 0, 0), return "@tris" if( vec( $vec, $num, 1 ) )
+;
    my $i = $num-1;
    while( $i > 0 )
    {
# print "<i $i>\t<s $sum>\t<t @tris>$/"; # UNCOMMENT TO SEE WHAT'S GOI
+NG ON...
        if( scalar @tris >= 3 ) {
            $i = $tris[0] - 1;
            @tris = ();
            $sum = 0;
        }
        if( vec( $vec, $i, 1 ) )
        {
            push @tris, $i;
            $sum += $i;
            $i = $num - $sum;
            next;
        }
        else
        {
            $i--;
        }
    }
    if( $sum == $num )
    {
        push(@tris, 0) while @tris < 3;
        return "@tris";
    }
    return "not found!";
}
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Update: changes as per YuckFoo (below)

Update 2: always returns an array of length three (below)

~Particle ;Þ