This UPA article mentions the same dosage.
I'm not sure if the 1999 copyright is the article or the UPA website?
http://www.unitedpeafowlassociation.org/MoreonWormingandDisease.html
This is the earliest mention of 3cc/gallon I see on BYC, and this person attributes it to Deerman:
https://www.backyardchickens.com/t/118688/peacock-lethargic-not-feeling-well-but-no-outward-signs-of-sickness
and here is Deerman saying the same...