Then, near it, build a Level 1 Slow Tower of that exact same type.
Creeps passing through both (as long as the Level 1 Slow refreshes the Level 3 Slow) will be permanently slowed down to 90 movement speed. This remains true even once they leave the slows.
Doesn't seem to work when trying with Level 2 & Level 1, so it's probably a coding error with just Level 3's interactions.
How to permanently slow creeps down:
Doesn't seem to work when trying with Level 2 & Level 1, so it's probably a coding error with just Level 3's interactions.