The four-digit number 1210 has an interesting property.
The first digit, 1, counts how many zeros are in the number.
The second digit, 2, counts how many ones are in the number.
The third digit, 1, counts how many twos are in the number.
The fourth digit, 0, counts how many threes are in the number.
Another number with this property is 2020 ( 2 zeros, 2 twos, no ones or threes.) Now, make a 7 digit number ABCDEFG so that the first digit, A, counts the number of zeros in the number, the second digit, B, counts the number of ones, ans so on down to G, which counts the number of sixes in the number.
+3 ABCDEFG = 3211000
As you said, A counts the number of zeroes, and there are 3 zeroes (E, F, G), so A = 3.
B counts the number of ones, and there are 2 ones (C, D), so B = 2.
C counts the number of twos, and there is 1 two (B), so C = 1.
D counts the number of threes, and there is 1 three (A), so D = 1.
There is no four, five, six in, so E, F, G are all zeroes.
That's my opinion 
+3 ABCDEFG = 3211000
As you said, A counts the number of zeroes, and there are 3 zeroes (E, F, G), so A = 3.
B counts the number of ones, and there are 2 ones (C, D), so B = 2.
C counts the number of twos, and there is 1 two (B), so C = 1.
D counts the number of threes, and there is 1 three (A), so D = 1.
There is no four, five, six in, so E, F, G are all zeroes.
That's my opinion
