Hi,
You just need to write 3 lines as below in the java script. and its working fine.
window.open("http://www.africau.edu/images/default/sample.pdf",'_blank');
window.open("http://www.google.com",'_blank');
window.open("http://www.africau.edu/images/default/sample.pdf",'_blank');

Also if still not opening 3 tabs then, you need to check browser because browser blocks that as popup. So please ensure there no such message/warning from browser.
Regards