If the third digit of an acceptable product code is not 0, which one of the following must be true?

It seems like 3 could be 3rd if the 3rd number is not zero given that's the only other rule we are given. If the 2nd number is double the first, it could be 1 then 2. Then if the third number was 3 and then fourth was zero the number 4 could be 5th. So, why can't 3 be the third number as well as 0 being the fourth number? What did I miss, because I keep getting these wrong and can never figure out why. It's my biggest weakness in the lsat and often the section with the least explanations in study.