There are 3 different stackexchange sites that deal with this subject (math/cs (computer science)/cstheory (theoretical computer science). Is there a way to ask same question on all stackexcahnged at once? If not, which stackexchange you suggest I use?
It is poor form to post the same question on multiple sites. Post it on one. If you wait a few days and don't get any answers, then it is generally considered okay to post it on a different site, but I have seen the recommendation of posting a link on each to the other one.
At Math Stackexchange, we have many discrete math questions and many people who can answer them. Of the sites you listed, this site is the biggest by far, so you will probably get your answer quicker here.
For example, we have 1284 questions tagged graph-theory, which is a small subset of discrete mathematics. The computer science site has 1327 questions TOTAL.