Developer Edition comes with two full Salesforce licenses which provide access to all of the standard objects such as account, opportunity etc.
If you have created three users then at least one of these will not be able to access some standard objects, depending on the license you chose when you created the user.
Salesforce platform licenses can use accounts and contacts, but none of the other standard objects like opportunities and leads. Other licenses are more restrictive.
To find out the capabilities of the user license that you have chosen, navigate to Setup -> Manage Users -> Users, click through the record for the user in question and find the license on the top right:
and then check the license capabilities at:
There may also be restrictions for the profile that you have selected for the user - to check this, click the Profile field link (below the User License in the screen shot) and check the permissions for the object(s) you wish to access. Note that the profile cannot override the license restrictions, so if a license doesn't provide access to an sobject, it won't appear in the profile.
If you are a Salesforce partner you can sign up for a partner developer edition which comes with more licenses.