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You will lose some link authority that comes with 301 redirects but if the new domain is worth the branding and potential gain of future backlinks, you should go for it. As BeardoCo said, you need to replicate/recreate the posts from the first two sites onto the third and have individual, page specific redirects set up. This would conserve as much relevancy and link authority as possible. Also take note that your traffic will most likely tank for 2 weeks while G deals with the site transition. Don't panic, rankings will return within 3 weeks. Make sure you tell google you changed domains and set up 301 redirects as soon as you do that. Otherwise, you can get hit with a penalty for creating too many pages too quickly + content is duplicate from the other two sites. Good luck!