The statute, 23 MRSA 3101, requires that, "The determination of each owner's share of the total cost must be fair and equitable ..."
It seems to me that it would not be fair and equitable for those who use the road with passenger cars to contribute the same amount to maintenance as those who use the road with heavy trucks. The Courts have so far interpreted "fair and equitable" to mean that whatever your formula is, you have to apply that formula in the same way to everyone. So if your formula is that everyone pays the same amount, the Court would approve it. But many road associations charge different rates for different levels of use, i.e. undeveloped lots contribute less than residential lots, year-round residents contribute more than seasonal residents, etc. As long as that formula is applied the same way to everyone in each category, that would also be considered fair and equitable.
Your membership could vote to change your formula to say that commercial properties pay more than residential properties. If you had just one gravel pit, the owner might be able to argue that it was unfair for there to be a different rate that applied only to that owner. But where you have three, if all three are charged at a higher rate, I don't see how they could say that was unfair. Then the question becomes how to determine what's a fair share for the owners of the gravel pits, in terms of what proportion of wear and tear on the road is due to their use as opposed to residential use.