Depending on what bullet I'm using, I do use 3fg 777 in my .50cal Knight USAK In-line. I can't say that I've found it any cleaner than 2fg though - I've not shot just the 3fg exclusively on any given day to be able to tell on clean-up.
For the record, what I've found in my rifle is that the 3fg works better with heavy sabots (350gr & up), while 2fg is more accurate with any weight conicals & PowerBelts, and also with sabots of 300gr or less.
But all these rifles are different critters, so I'd say give it a try and see what it does - you may be pleasantly surprised. :-)