When you set up a Channel, there is one owner of that channel. This is the social network account that will be posted to. If the other two users log in with their Joomla credentials and have the correct permissions for 'Post on Social Network' (found in the JFBConnect backend > Overview page > Options toolbar button), their post will go to the Channel Owner's account. There is not a need to add a separate Social User.
Each social network will need its own channel and the channel owner will have to authenticate for that social network. For instance, if you want to post to a Twitter Stream, you'll need to log in with Twitter on the front-end and then create a Twitter channel with that owner.
By the way, Google+ is not a supported channel type. We allow posting to Facebook Pages/Groups/Profiles, LinkedIn Company Streams and Twitter feeds with our Channels feeds.
I would suggest reviewing our
Quick Start Guide
for "Post Content to Social Networks" to get an overview of how the process works. But let us know if you need more clarification or help.
-Melissa