Problem is time lag & liquidity
My experience says use the A & B funds, since you must be in them with your prov fund bucks. Also use the freebie meetings to hear another point of view.
However (comma) the C fund is just too cumbersome & slow in today's markets. You need to be nimble just to retain what you've got. The days of "buy & hold" are toast. If you limit yourself to the C fund, you are simply leaving your money on the table all night. Any poker player will tell you that's a disaster - just a matter of when you get tapped.
IMHO, take any bucks destined for the C fund, find a good broker (Interactive is OK) & park it there. Figure out how to invest it properly & do so. You are ultimately in charge of your own $.