I have one canonical web site, http://example.com.  I also got the .net and .org, domains, so I have 301 redirected all the following to my canonical web site:http://www.example.comhttp://example.net, http://www.example.nethttp://example.org, http://www.example.orgIn Google Webmaster Tools, I specified http://example.com as the preferred domain.All of the above works fine.  Now the question:The instructions for Google Webmaster Tools implies that I should register all six sites with Google Webmaster Tools.  I would think that registering the one canonical web site is adequate.  Which is it? 
Add all 301 redirected sites to Google Webmaster Tools?
google search console;301 redirect
It depends on the current situation and on what you want to do with it.If .net and .org have no pages indexed in Google and you're not planning to have any indexed it's fine as is. No need to add your non-canonical domains to Google Webmaster.But if the .net and .org domains have already been indexed and you want to redirect this and indicate to Google that .com is your canonical domain name now it's a good idea to also register the .net and .org domains. With that you can let Google know that .com is the domain name it should show.See also my answer here: SEO much better for domain that 301s to other