In a bowl, whisk together mayo, sour cream, and buttermilk. Pull one large chile out of the can of chipotle peppers in adobo sauce (you can find these in the ethnic food aisle). Slice it down the center and de-seed if you are like me and are a little wimpy when it comes to spicy foods. If you like spice, leave the seeds. Scrape the guts away from the skin and discard skin. Add to the mayo mixture, whisking well. Mix in cumin, garlic, and oregano. Add salt to taste. Thin out with more buttermilk if you prefer it thinner.