How to open different URLs by checkbox?

This is my HTML and JavaScript code

<input type="text" id="text" /> 
<input type="button" id="btn" value="Submit" onClick="javascript: window.open('http://example.com/' + document.getElementById('text').value);" />

If you input anytext in the form box and click on the Submit button, it will open http://ift.tt/2x2rx9K. I want to add three checkbox named page1 page2 and page3 under the form. If I click on the checkbox page1 and click on submit button, it will open http://ift.tt/2vOPL93

How can I do it?