Sorry to hear you're having troubles there. Perhaps the docs aren't quite as clear as they should be, but generally if you're doing things on the command line it's much better to use composer to sort out dependency management with installing the base software and modules. This quick guide will get you started with the framework, CMS and blog, widgets and comments modules. Assuming you're on a *nix style operating system and have command line access:
First of all, install Composer:
curl -sS https://getcomposer.org/installer | php mv composer.phar /usr/local/bin/composer
Then install the code using composer (change /var/www/ss3 to a location of your choosing in the webroot):
cd /var/www/ss3 composer create-project silverstripe/installer . composer require --no-update "silverstripe/blog:*" composer require --no-update "silverstripe/widgets:*" composer require --no-update "silverstripe/comments:*" composer update --no-dev
Enable widgets by adding these two lines of code to mysite/_config.php:
Object::add_extension('BlogTree', 'WidgetPageExtension'); Object::add_extension('BlogEntry', 'WidgetPageExtension');
Finally, open your browser and go to where you can access the site to start installer, e.g. http://localhost/ss3/install.php
More packages can be found on packagist.org and installed like above (search "silverstripe/" on the packagist site for examples).
If I wanted to add the userforms module, for example, I would execute the following commands (change directory to where you installed SilverStripe first):
composer require --no-update "silverstripe/userforms:*" composer update --no-dev
Now there are a lot of modules out there, and not all of them are compatible with 3.0.5, which is the latest stable version of SilverStripe at the time of this writing, however if the module is on packagist you're more likely to know the version it's *actually* compatible with.
Hope that helps.