Have nested repeaters
Open the tool with already 1 top level entry
Use the + button of the firt line 3 times to add 3 lines
Set values.
Save.

Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.Map.get(Object)" is null
java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.Map.get(Object)" is null