Creatures and NPCs have character levels, similar to your player character.
Command Creature to Level 3 means that if the creature is at level 3 or below and you have 100% spell effectiveness, then the spell will work. Otherwise nothing will happen.
There is no in-game way to tell the level of a creature (or NPC) without being on the PC and using the console. Also if you are wearing armor then the spell effectiveness is reduced. So a Command Level 3 spell might only work on level 2 creatures.
Command Creature up to Level 3 is a very weak spell.
However if you get a Command Creature Level 25 and 100% spell effectiveness, then the spell will work on all creatures, even if above level 25.