Criteria for backcourt violation
1. Team control (and initial player control when coming from a throw-in)
2. Ball gains frontcourt status
3. Team in control is the last to touch the ball before the ball gains a backcourt status.
4. Team in control is the first to touch the ball after the ball gains a backcourt status.
Team control is establish by holding or dribbling a live ball (including a throw-in). Team control continues while a team is passing the ball and only ends when there's a try, the ball becomes dead or the other team gains possession of the ball.
The other important part, which seems to be the part you're confused about is player and ball location. The ball has the same status as the player when it touches or is control by a player. The ball is considered in the frontcourt when it touches anything in the frontcourt (player, floor, or official). The ball is considered in the frontcourt when a dribbler has both feet and the ball in the front court (3 points rule is only for a dribbler).
A player is considered in the backcourt when any part of his person is touching the backcourt including the division line. An airborne player maintains the same status/location as he/she had when he/she left the court until the player lands again. So a player jumping from the frontcourt to the backcourt is still considered in the frontcourt until the player lands.