It looks like 3 is a quadratic residue because both 5 and 6 can be squared to give 3 mod 11. But then we know half of the elements will be quadratic residues and the other half non-residues. So there should be two elements when they are squared that give 3.
Another way of looking at this is to ask what solutions are there to the following polynomial: x2 - 3 = 0 (mod 11) There should be 2 solutions and there are.
When we are looking at the residue classes mod 11, we aren't looking at integers any more. Instead we are working with equivalence classes of integers, or sets of integers. The integers would be in the integral domain Z, where there are primes because multiplicative inverses for all elements do not exist, but these residue classes are in the finite field Z/Z11 where there aren't any primes anymore. In the finite field, the non-zero elements all have multiplicative inverses and so they would be units like 1 and -1 are in the integers Z. For example let n, not equal to 0, be an element in Z/Z11, then since n11-1 = 1 (mod 11), the multiplicative inverse of n is n9 (mod 11).
I got an account on https://math.stackexchange.com/ to get more information as well. It is good to have questions. The available answers aren't all the answers. Although QR is useful, what we really want is a quick way to evaluate (p/q) without having to consider (q/p). Quadratic reciprocity allows us to evaluate the one that is easiest to calculate, but perhaps there is a faster method. That sounds to me like what you are looking for.
That is an interesting question. One doesn't have to take the representatives for the equivalence classes from {0,1,...,p-1}. They could come from {-(p-1)/2,...,(p-1)/2}. The evenness and oddness of the result might change when using that set.

