Cases : better using sub-directories or sub-domains?

Hi, I am a novice in web and internet matter, I am building a fresh new site, I plan the main contents to be : News, forum, and classified ads. I don’t understand what would be the best way to place or how to arrange those contents in the web structure or internet, whether :

  1. To create a front/landing page that has menus linked to :
    a) 3 domain : ie., and
    b) 3 sub-domain : ie., and
    c) 3 sub-directories : ie,,

  2. What best structure to choose among those mentioned above related to :
    a) Security issues
    b) Easier or lighter or faster for the Server to serve heavy traffic
    c) SEO and Page rank