A script. Effectively you have an item that starts at 3 charges. Once you use a charge, every 18 seconds a charge is restored until it registers it is at 3 charges again and then the script stops and waits until it gets used again.
The net result is that you can use the item every 3 rounds in general plus you can save up to 3 charges to release more rapidly if you're in a bad situation. May bump it up to restoring a charge every 24 seconds depending on how testing goes, but that's just fiddling with tuning at that point.
Take a look at the include for auras in the mod I gave you. It does pretty much precisely what you want, along with another include for item charge alteration that properly displays on the quickbar.
As for TMI looping, take a look at the old, rather than the new, code for HGLL feat-table building. It shows how to do stacked loops with variables to avoid TMIs.
Funky