I finally broke down and got a 3rd retainer via Mog Station. I got the email that the transaction went through and it is reflected in my account on Mog Station.
http://i.imgur.com/uV6GNSX.jpg
I just went to hire the 3rd retainer... and the game tells me that I'm currently only allowed to have 2. When will I be able to get the 3rd retainer? My inventory is too full!