On a polygonal barrel, they're not really "grooves"; more like "lobes". Ever look at a Glock barrel? I think they're a little easier to really observe in a shorter length barrel. Not sure sending a barrel into a rotary forging hammer mill matters whether it's a 3, 5 or more "lobed" bore. Maybe the owner of Black Hole Weaponry can answer the question.
From a standpoint of sheer physics, trying to "deform" a copper-jacketed bullet - by explosive force - down a bore with less contact area seems the most logical. And - I'm guessing - with only 3 "open" spaces around the bullet, there's less space to have to seal.
I keep hearing about increased muzzle velocities - using identical rounds - in favor of a polygonal barrel vs. traditional Enfield" rifling. That and the dramatically reduced copper / lead fouling - i.e. easier cleaning - in the bore is what sold me on going to a poly design.
Mine shoot VERY accurately. Have no way - or desire - to gauge velocity. I pull the trigger, BANG! - and a hole appears in my target.
:D