The proposed system of 4d6k3 reroll 1s and 2s indefinitely its the exact same thing as rolling 4d4k3 +6, which means each score on average will roughly be about 2.5 to 3 points higher (haven't run the numbers though, but that's my guess, ill see if I can whip them out fast, if not im prolly not gonna do it)

My table usually does 2 sets of 4d6k3, pick whichever set you want, if you don't like any of those you can do a 3rd set but must use it.

Over the years we've tried some variations of this, the only other that I remember popping up more than once was 2d6+6 but scores are in order (1st roll is STR, 2nd roll is DEX, etc)