public HashSet<BigInteger> getFamily3(BigInteger n, int[] positions) {
        char[] bi = n.toString().toCharArray();
        HashSet<BigInteger> prs = new HashSet<BigInteger>();
        if (positions.length == 3) {
            for (int i = 1; i <= 9; i++) {
                for (int k : positions) {
                    bi[k] = (String.valueOf(i)).charAt(0);      //substring(k, k+1);
                }
                if (fromFromChars(bi).isProbablePrime(20)) {
                    prs.add(fromFromChars(bi));
                }
            }
        }
        return prs;
    }