I’m not aware of a way to send to Instapaper via right click (in Reeder v3). I’ve always preferred keyboard shortcuts and set up Reeder to allow me to run through 200-300 stories in a relatively short period of time.
J moves down 1 story, K moves up, B opens the story in Safari (in the background), command+R adds the story to the Safari Reading List, and command+I saves the story to Instapaper.
When I’m done sorting, the articles I want to read immediately are open in Safari, those I want to read next are in the Reading List, long reads, research, etc. are in Instapaper.
Instapaper offers a Safari Extension but I prefer using the Instapaper bookmarklet to save articles since it works the same in Safari, Chrome, Brave, etc. There is also an Instapaper Text bookmarklet that provides a “Reader” mode in most browsers: