I disagree that these are mistakes. They are, in fact, correct in certain dialects. When it is a "common mistake" in an area, it is not a mistake, it is merely a change in the language. Language evolves. Language has, is, and always will evolve. Just because one dialect is different than yours doesn't make it wrong.

That said, I do not teach these three "mistakes," but I do hear it very often in the area I come from because it is part of a regional dialect. When I am confronted with something like this, or if I have a great opportunity to bring it up, I do point out these differences in dialects. I don't say any are more "correct" because none are more "correct" than the other, I let the student decide what dialect they prefer. I've had students that prefer British English and I have no problem with it, I help them recognize these issues, and "correct" it for their preferred dialect. It is much better than taking the <em>prescriptivist approach and saying,</em> "This is the correct dialect and all else is wrong, including millions of native speakers that make these 'mistakes.'"