You can add in the form ALL the parameters of the PayPal button: amount, item descriptin, item nr., amount, currency, etc. The code has to be changed acordingly, and the form fields must reflect the code elements. So, change the button code to :
Code:
<?
if ($amount != 0){
?>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="you@yoursite.com">
<input type="hidden" name="item_name" value="="<? echo $_POST['item_name']; ?>">
<input type="hidden" name="item_number" value="">
<input type="hidden" name="amount" value="="<? echo $_POST['amount']'; ?>">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return" value="http://www.yoursite.com/returnpage.php">
<input type="hidden" name="currency_code" value="<? echo $_POST['currency'];?> ">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-BuyNowBF">
<input type="image" src="https://www.paypal.com/en_US/i/btn/x-click-but02.gif" border="0" name="submit" alt="Make your payments with PayPal. It is free, secure, effective.">
<img alt="" border="0" src="https://www.paypal.com/it_IT/i/scr/pixel.gif" width="1" height="1">
</form>
<?
}
?> and include, in the form, a drop down with the thre values : USD, GBP, EUR for the three currencies you need.