What is a Zabla?
A Zabla stands for the words “Zeh Borer Lo Echad/Each one chooses one.” A Zabla is an alternative to a set Beis Din, if for whatever reason the sides are unable or unwilling to take their issue to the local Beis Din. It works as follows: Each side picks one judge of his choice and the two chosen judges then pick a third judge, for the three person Beis Din. All three judges have equal power and together judge the case and bring out a justified verdict.
 Michaber C.M. 13/1; Mishneh Sanhedrin 23a following opinion of Chachamim
Other opinions in Gemara: Rebbe Meir holds that the two sides get to decide and pick a third judge.