I am considering making my website not focused on classifieds but also with my Blog and possibly a forum

What I would want to do is just make a main entrance page that showed off all three. How would I go about doing this? Would I need to move the osclass script to a different directory? Or could I just modify the theme where it showed "widgets" of all three?

@Anthony Marshall
I would not recommend creating navigation page like home page, rather to create block where you can switch, maybe in header, or complete section on home page.