I wrote to Plus 3 yesterday asking if there was a way to use nicknames instead of one's real name, and here's the response I received this morning:
Hi Joanne,
Welcome to Plus 3 Network!
Yes, you can use a nickname if you prefer to not have your full name on the site. When filling out the profile information page, just put something else in for First Name and Last Name. It might not be the most elegant solution right now, but that is the only way around the issue.
See you around the network!
I'm still OK with using my real name but for those of you who are hesitant, there you have it.