In which case does PLOS ONE use a third reviewer? We have submitted our manuscript 10 weeks ago to PLOS ONE and are still waiting for a first decision. 
We sent an inquiry, and they replied that the paper is under review with a 3rd reviewer. Does it mean there is a split decision? How much longer can it take to receive their response?
It is my first manuscript submitted for me, it is on cancer biology (leukemia) and it feels very frustrating.

The email reply was:


  "Thank you for your submission to PLOS ONE.
  
  I would like to take this opportunity to update you with regards to the current status of your manuscript, as we are aware that it is taking longer than we anticipated for you to receive a decision on your submission.
  
  As you know, we always strive to provide authors with decisions as quickly as possible. Unfortunately, this is not always possible especially when it proves necessary to consult external referees. In regards to your manuscript PONE-D-15-0812 it appears that the comments from 2 reviewers have been submitted, and we are waiting for the other review which is currently outstanding. We are aware of this delay, and are following up with the relevant individual. Once all the reviews are submitted, the Academic Editor will be able to make a decision regarding your manuscript, and inform you of their decision.
  
  We appreciate your patience, and hope you will contact us if you have any questions or concerns.
  
  Kind Regards,
  Tresa
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).