If It Ain`t Broke, Don`t Fix It

I know that if my website just starts building blog contents, posting content in one single domain is better then posting articles on have multiple domains.But, what if my clients already have multiple blogs for several years and each of them already established their own authority and ranked for similar keywords? For example, my clients have: blog.example.com blog.example2.com example.com/blog And each of them is currently performing well on the SERPs for similar Keywords. Would consolidate all three domain`s content into one domain hurt or help my SEO, and how do I choose which domain to be "the one" that survive and consolidated other domains` contents to it. Let me know your thought or experiences, Thanks