Asking for a Fourth Review I have submitted a paper to a conference with a rebuttal phase.
I have received the first round of reviews, and should be writing 
an answer which addresses issues raised by the reviewers. 

I have received two very positive evaluations, with one accept and 
one weak accept. The third review however was -2 (reject). 

The reviewer that rejected the paper did so under the argument that the 
paper defined a nice problem, but "without interest for the community 
of the conference". He also tried to shift the focus of the paper from 
the main result to a very auxiliary result by treating the auxiliary result 
as if it were the main. 

1) Would it be worth to send an email to the conference chairs asking for 
a fourth review? 

2) What would be the likelihood that the chairs would do that?

3) In case the fourth review is positive, would it have the potential to be taken into consideration when accepting/rejecting the paper?
I don't think it's worth it. I've seen conference program committees accept papers with such scores many times. I think if you address this reviewer's concerns, perhaps offering some changes in the introductory parts to reemphasize the main result, you've got a better chance of acceptance than if you ask for special treatment.