How To Insert Many To Many Tabel Auto Increment - php

Question

A B C
------- --------- ------------
id_A* id_B* id_C*
name id_A** name
address id_C**
I created a table as in the example above I've tried to do input in Table A, B then C, but what if I want to do the input of C then the input table B in conjunction with Table A, all tables in the auto increment primary key
<input type="text" name="id_A" />
<input type="text" name="id_B" />
<input type="text" name="name" />
<input type="text" name="address" />
<select class="form-control" name="id_C">
<?php
$query = "select * from C";
$r = mysql_query($query);
while ($row=mysql_fetch_array($r)) {
?>
<option value="<?php echo $row['id_C'];?>" name="id_C"><?php echo $row['name'];?></option>
<?php
}
?>
</select>
And Then
isset($_POST['submit'])) {
$id_A = $_POST['id_A'];
$name = $_POST['name'];
$address = $_POST['address'];
$q = "insert into A (id_A,name,address) VALUES('$id_A','$name','$address')";
$dq = mysql_query($q);
$id_B = $_POST['id_B'];
$query = mysql_query("insert into B(id_b,id_A,id_C) values('$id_b', '$id_A' ,'$id_C')");
please help me,
i' am a realy2 confuse