First off, 2 pair of fingers is 4 fingers, so it's pretty hard to figure it out what you're saying.
This falls into one of those gray areas. In my opinion, it's fine. Also, 3 fingers are very hard to use due to the 2 key limitation in standard (only 2 buttons can be pressed at the same time, even though you have 4 buttons available to press), so even if you did learn how to play with 3 fingers, it's very unlikely that you'd be any good and 2 fingers are likely going to be better.
When I switched from ring+middle finger to index+middle thought my streaming speed increased dramatically because your ring finger and middle finger share the same tendon.
If you have to ask if something is considered cheating, and you have to use an external program to achieve that effect, it probably is
while the rules have this to say about macros, it's important to note that you can purchase a keyboard with a mouse key fairly easily.
The macros that aren't ok are the ones which constantly hit in time or the ones that play the game for you and remove the human element.
Im not gonna play with 3 fingers, im just trying to be able not to go switch in options all the time, 3 keys would probably be pretty hard yes indeed.