You can add them directly from the Posts module by clicking on the "Add an account" button in the left column.

Alternatively, go to "Social Accounts" and click on "Add an account".

Then select the Facebook, LinkedIn and Twitter accounts or pages you wish to add, with no limitation of volume.

You can add multiple Facebook and LinkedIn pages at the same time if they are all attached to the same account by checking the corresponding boxes in the list. For Twitter, accounts can be added one by one.