On the trial of any civil action to a jury, each party may challenge peremptorily three jurors. Where the court determines a unity of interest exists, several plaintiffs or several defendants may be considered as a single party for the purpose of making challenges, or the court may allow additional peremptory challenges and permit them to be exercised separately or jointly. For the purposes of this section, a “unity of interest” means that the interests of the several plaintiffs or of the several defendants are substantially similar. A unity of interest shall be found to exist among parties who are represented by the same attorney or law firm. In addition, there shall be a presumption that a unity of interest exists among parties where no cross claims or apportionment complaints have been filed against one another. In all civil actions, the total number of peremptory challenges allowed to the plaintiff or plaintiffs shall not exceed twice the number of peremptory challenges allowed to the defendant or defendants, and the total number of peremptory challenges allowed to the defendant or defendants shall not exceed twice the number of peremptory challenges allowed to the plaintiff or plaintiffs.