What do the first three digits in binary code mean?

I am not sure what you are asking.

Binary numbers can be 0 or 1 or 10 or 110 or 1000001011. The first three digits are certainly not useless in any of these examples. Every digit in a binary number is a normal digit.

Is there a chance you are talking about some character encoding? Please be more specific in your question.

http://sticksandstones.kstrom.com/appen.html

Is it possible the question refers to the older than dirt punched cards?

Should have read CI's link first. It sounds more likely.