Need advice re: multi-blog or hack just one

I have a project where I need to have normal pages, like Home, About Us, etc. but then I basically need 3 sets of parent and child pages.

One will be for the blog.
Another for projects.
And one for links.

The child pages will be duplicated many times and all of them should categorize properly into their respective parent page.

So, you wouldn’t see blog posts or pages anywhere in the projects parent or child pages or the links parent or child pages.

I’m not sure how to go about this with one installation, because of the possible need for 3 different themes and 3 separate chunks of pages. Would it be better to install 3 instances of WordPress on the same server and just manually create a static top nav for the blog, projects and links and then manage the 3 through one admin panel?

Or can it easily be done with just one installation of WP?

Thanks for any help you can give me.