Well how is it different? 3 fly uses 1 less coin so they are different? Thats like selling a trick and claiming it's different because you use different colored cards or something.
If you don't know the difference between Three Fly and Coins Across, you shouldn't be having this argument! And no, that's definitely NOT the difference.
Three Fly happens at chest level, making it a better, in my opinion, because they get to see and remember your face. Look at Daryl's Three Fly III, and notice that he has the coins at chest level.
On the contrary, Coins Across is generally done with your hands near your belt. Open hands or closed hands, it makes no difference. Look at any Coins Across and notice where there hand is.
And THAT'S the difference between Three Fly and Coins Across.