Ans: They need not worry: the number of coins left for them is always divisible by 3 (assuming that the grandmother left behind more than 2 coins).
Let the number of coins in each pot be n. Then there were n pots in each room, and the house had n rooms, thus making a total of n^3 rooms. The housekeeper got one pot and thus n coins.
That left n^3-n coins for the grandchildren. Note that: n^3-n=n(n^2-1)=n(n+1)(n-1).
We have three consecutive numbers and hence definitely one of them is a multiple of three. Thus the total number of coins is divisible by 3.