Nyocca,
Very sorry for the delayed response.
Integrating all 3 registrations shouldn't be a problem. Really, JomSocial, K2, and JLinked each create Joomla users. Once those user's are created, they 'extend' the user by adding some additional information into their own tables. This is completely normal, and underneath, every user is a Joomla user. There may be times when you want to disable some of the other methods of registration, but they shouldn't conflict with each other if they are all used at once.
The "Invalid Token" message is caused by how Joomla does some security checks. When a user views the login page on your site, Joomla creates a unique ID (token) for that user. Then, when they submit the form to login, that token is checked to make sure the same token is submitted that was generated for the user. The security reasoning behind this isn't important, the problem is that the tokens aren't matching for you. There's a few things to check that can be causing this problem:
1) In the Global Configuration area of Joomla, set the "Session Timeout" to a high number, like 60. That means that the token generated will be valid for 60 minutes. If the number is too small, the token will expire if the user is simply sitting on the login page too long.
2) Disable any caching or optimization extensions you have enabled. This includes Joomla's cache in the Global Configuration or the System - Cache plugin. While these shouldn't be a problem, try disabling them, clear the Joomla Cache and try again. If it starts to work, then try enabling them one-by-one and and see what one is the problem setting.
Hope that helps get you started, but please let us know if you need anymore information.
Thanks,
Alex
Invalid Token
