Domain redirect question.

I wanna register 3 NEW similar domains eg. www.coffee.com www.creamer.com www.sugar.com but I want coffee to be the main site, and the other two just to redirect to coffee. Now if I use a 301 redirect and have nice keywords and etc. Does everytime I update www.coffee.com with new content does the other two get better SEO or do I have to work on the SEO with them too?