FWIW ... used Flatworm Exit yesterday, followed directions to the letter (was actually light on the carbon).... had them primarily in one corner of 180 gal tank... it wiped out >95% of them with no downside that I can detect. Pods still scurrying around at night. Plan a second treatment next week (higher dosage) to hopefully knock out the remainder.
You are probably right on a "3rd treatment". Did a second treatment (150% dose) Monday afternoon, and checking the glass this afternoon ... saw two. Siphoned them off, and will keep an eye on it, but probably heading for another round at some point.