And no, I will not be using category labels or tags to parse the posts for visitors. That option is out. Don't even ask me why. It just is.
oh, ok... that was exactly what I was going to suggest especially as there are so many great plugins that make managing categories and tags a breeze both for visitors and search engines but I won't ask :)
Having 3 separate blog installations is going to be a webmastering nightmare. Imagine customising, updating and tweaking a blog and then repeating it all on two other blogs.
I think the answers is in how related those topics are. If a visitor was interested in topic A but he ended up landing on topic B. Would he bounce or would he still read the article on topic B? Then, considering you're going to ditch categories and tags (which means you will struggle to serve meaningful "related posts" to a visitor) will the visitor be able to navigate to topic A - which is what he's mainly interested in?
As far as search engines are concerned, there's very little difference between having
(unless you mess something up with the installation, permalinks etc etc)
So, for me, the best advice would be - think about the visitors and forget the search engines.