In which case does PLOS ONE use a third reviewer?
I'm not certain what PLoS ONE's exact criteria are for number of reviewers (and there is likely to be some discretion allowed for individual editors as well), but in most venues that I have encountered, there is a preference for having at least 3-4 reviews to increase both the breadth of opinions and the likelihood of "consensus" on a paper, and often a required minimum of 3 reviews.  So most likely 3 reviewers were invited from the beginning.

Moreover, the asynchrony of journal reviewing means that it is very easy to end up waiting for a final review.  The typically process for managing journal reviewing is something along the lines of:


Invite a set of reviewers, who have 1-2 weeks to respond to the invitation.
If some reviewers decline or fail to respond, invite more reviewers and iterate until a sufficient set accept.
Once reviewers accept, they have 2-6 weeks to return the review.  Many reviewers put off responding to the last minute, and some ask for extensions.
One or more reviewers may fail to return a review at all, at which point the editor puts a black mark on them in the system and has to start all over again inviting a new reviewer.


You can see how this can quickly add up to 10 weeks or more, and it can stretch even further if the editor isn't acting immediately upon events in the reviewing process (as often happens).  

So take a deep breath and don't worry yet: there is nothing you can really predict about your paper based on the current timing, since you're well within the normal range of journal review time (unlike the request for revisions I got today for a paper submitted 14 months ago).