I don't think we'll ever know everything that happened. 3,000,000 votes is a big number though, so I can't draw any reasonable conclusions that the DNC swung the election to Hillary by themselves. We do know that some of them tried, however. That's not acceptable to me, no matter the impact it had.
One of the other problems is the super delegates. There were quite a few states that Bernie won (some by large margins), and yet Hillary walked away with the majority of the delegates from those states. What exactly is the point of voting when super delegates can pick the winner no matter what the people have to say? The DNC, if nothing else, should exist to get out the vote. Do they not see how a rigged primary process where party elites trump the will of the people will have the opposite effect? People feel disenfranchised when they take the time to vote, their horse "wins", only to be told by a handful of super delegates that they know better.