Yes, your machine will run a third account just fine. Back in the day we used to run 3 accounts on much worse

If you have performance issues, you will want to lower graphics settings in the game to increase performance at the cost of some graphical prettiness.
However, WinEQ 2's picture-in-picture will only manage a single PIP window.
Being that you are a subscriber, I would highly recommend grabbing ISBoxer Toolkit (used with Inner Space) to take your game to the next level. It can manage as many PIP windows you like or let you configure your game windows in any desired fashion via a click and drag interface. It can also manage your ability to send keystrokes or mouse clicks to the other windows, among other features. Definitely give it a look, you lose nothing by trying it out
Also, if you need the EQPlayNice rendering limiting feature (which drastically improves performance BUT does the "flickering" that some people prefer not to have) when using ISBoxer, you can leave WinEQ 2 running in the system tray, and disable the FPS limiting features in ISBoxer (uncheck the boxes).