How to Spawn a Mob With a random level?

Mob attempt:

Tried adding

Level: 3-10
Doesn't Work
So tried something a lil more complex but the trigger doesn't work.
All it does though is set the mob to the deafult level of 1

Zombielvl:
MobType: zombie
Health: 100
Damage: 10
Display: '&5Zombie Lvl - <mob.level>'
Options:
MovementSpeed: 0.3
Drops:
- GOLD_NUGGET 1 1
DropsPerLevel:
- GOLD_NUGGET 1 0.5
LevelModifiers:
Health: 5
Damage: 0.5
Skills:
- setlevel{a=set;l=<random.3-10>} ~onSpawn