I have traded C recently, and find that a very large % of prints are coded third market, instead of nms, nyse, or pse. I assume NITE prints there, and know edgx/a/ise does, but I can watch literally millions of shares print third market (far more than the displayed total of shares at that price) at my limit and still not be lifted. At present I have access to nite/citi/arca/nsdq/edgx-a, but it seems clear I am missing a major liquidity venue. What are the major volume sources for thrd? Is there anyplace that gives this data for listed stocks? I am trying to see what liquidity sources I could try to use in addition to what I have. Any thoughts?