Why do most teams have two males and one female? Almost all teams in Naruto have 1 female and 2 male members. For example, Kakashi's team (Kakashi, Obito, Rin) and Jiraya's students (Nagato, Konan, Yahiko). Is there any rule behind this?
I don't think there is any rule behind this because the main motive behind this is to test the dynamics of teamwork but there is one rule that is to how to team-up Genin:


  Genin are put in four-man cells, or squad, consisting of three genin
  and a jōnin-sensei, in order to learn teamwork and experience true
  ninja life under the supervision and protection of an elite shinobi.
   The make-up of these teams is based on the individual skills of the
   genin, so that there will be a balance between the teams. For
  instance, Naruto Uzumaki, who passed with the lowest grades, was put
  in a team with Sasuke Uchiha and Sakura Haruno, who passed with the
  highest grades and highest written exam scores, respectively.


So there is nothing states that there has to be one girl in every team. But to create a perfect balance between all teams they might have assign one girl in every team.