SEO and dividing part of website (e-shop, forum and website itself)

[rafnews]

Hi,

i'm facing an interesting challenge redeveloping a website that includes the website itself, a forum and an e-shop.
Basically i was thinking to develop those parts into 3 phases and thus to be independent of each other.

so the website itself will be stored under main domain (ex: www.example.com), while forums will be stored under a subdomain (forums.example.com) and e-shop as another subdomain (eshop.example.com)

i would like to know if this is a problem for a good SEO ?
or is it much more better to have everything under 1 "roof" www.example.com (main site), www.example.com/forums (forums) and www.example.com/eshop (eshop) ?

Please detail your answer in order to help me to understand.
thx a lot.