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This is a common practice, though in my experience it is better (and a LOT more natural) to buy other domain name extensions to protect your brand, and 301 redirect them to their .com counterpart.
This is what Google, Yahoo, and Bing do, so it's more than likely your best bet.
My apologies, I didn't catch the DSEO part.
In order to stack the 3 listings, a 301 redirect would not help.
That said, in order to truly make use of each domain, be sure to include undeniably unique content, preferably specific to the nature of the domain's extension.
e.g. The .org could be a listing of organizations your company is a part of, non-profits to which you have donated, events you've sponsored, and any other charitable work you're a part of.
As for the .net, you could focus purely on the history of your company and staff, with some helpful content about your brand (YouTube videos you've produced, slideshare presentations, twitter feed, etc.)
I hope that helps!
In my experience with content management solutions, it's easier (and thus more common) to keep all blog posts under the blog directory, then use categories in the URL and/or the Title tag (if necessary at all).
That said, most of the time I find categories to be more useful for users, and less helpful for engines.
So, to me the pros of using /category/content-title would only apply if the categories are very straightforward and short (which depends on your industry).
For example, if you run a outdoor hobbies blog, and you have clear, distinct categories like backpacking, camping, hiking, fishing, and so on, then using the category URL structure would be great.
However, if your blog is more specific (say backpacking only), your categories will start to become more convoluted and long, taking up a large part of the URL.
In addition, this can cause unnecessary redundancy in your URLs, which can appear like keyword stuffing.
Overall, I prefer to stick with /blog/content-title, and possibly include the category in the title tag if it's really important.