As an action, it's found by going to "Set Score," and in the "Points" variable, selecting "Custom" at the bottom of the list.
As a condition, it'll be recognized by "Score," and choosing "Custom" as the "Score type" variable.
With the action, you can add three to the Custom score at some point. if a player doesn't have a unit they should have, then have a trigger that says
== Player
- [number x]
== Condition
- Player [number x] brings "at most" "0" [unit y] to location "Anywhere"
- Player [number x] "Custom" score is "at least" "1"
== Action
- Set Score for player [number x] to "subtract" "1"
- Create "1" [unit y] at location [location z] for player [number x]
To show it on the leaderboard, you can have the following as an action of some trigger;
- Leaderboard (Points): Show Leader Board for most "Custom." Display label: '[label w]'