Normally with a PayPal standard implementation you specify the return page in this form element:
<input type="hidden" name="return" id="return" value="" />
You put in the URL of the page you would like the user to go to on return.
To get something in place like you are talking about you can use some client side js to populate the return field with the appropriate URL based on the user selection. The problem with the scenario you have described is using a checkbox, this is a problem since the user can check all three boxes and you only can have a single value for this element in the end.
Instead of a group of checkbox's you should use a radio group, this way the user can only select one. For the value of the radio group entries you can have the three different URL's. Then add in an onchange event that will make the value of the selected radio option the return hidden form element's value. This would result in have a page where the user can choose an option, then based on this option they have selected they are taken to a different page on return.
