$kacabol = 0;
$deger1=(int) get_post_meta($post->ID,"alan1",true);
$deger2=(int) get_post_meta($post->ID,"alan2",true);
$deger3=(int) get_post_meta($post->ID,"alan3",true);

if ($deger1 != 0) { $kacabol++; }
if ($deger2 != 0) { $kacabol++; }
if ($deger3 != 0) { $kacabol++; }

if ($kacabol != 0) {
 $ortalama = ($deger1 +  $deger2 +  $deger3) / $kacabol;
}else{
 $ortalama = 0;
}
echo $ortalama;