<?php

$tabela1 = array(1,2,3,4,5,6,7,8,9);
$tabela2 = array('a', 'b', 'c');

$cont = 1;
$aux = 0;

while(count($tabela1) > $aux){
if($cont <= 3){
echo $tabela1[$aux].'<br>';
$aux++;
}else{
echo $tabela2[0].'<br>';
array_shift($tabela2);
$cont = 0;
}

$cont++;
}