Hello,
To reinstall WSUS 3.0, we should make sure the old one is cleaned up. Please check whether WSUS service is still running on your computer, and have you remove Windows Internal Database(if your exchange also use WID, do not remove it).
You do not need to reinstall IIS, you can just remove WSUS related virtual directories under IIS manager.
Please follow article below to force remove WSUS 3.0:
https://windowshell.wordpress.com/2011/08/26/force-uninstall-of-windows-server-update-services-server-2008-rtm/
https://thesbsguy.com/2010/11/24/manually-removing-wsus-3-0/
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