What is the function of a third primer in a PCR mix?? - (Mar/05/2012 )

I'm currently carrying out a pcr reaction and the protocol recommends a mix containing three primers. I know there is a forward primer and a backward primer but if there's a third primer,what does it do? In real time pcr i think the third primer is a taqman probe which can be destroyed by the polymerase through it's 5' to 3' exonuclease activity in order for the reporter to fluoresce but I'm carrying out a non RT PCR and have 3 primers; HIST 1, HIST 2 and HIST 3 and was wondering HIST 3 function.

I have an idea about bridging but not clear enough. Any ideas out there???

Are you screening organisms for knockout/knockin of a particular gene by any chance?

Yes, actually screening eker rats for fibroid predisposition.