The Cabot Trail forms a complete loop, so it doesn't technically begin or end in any one spot. Good starting points would be either Baddeck, or Cheticamp (having come up the beautiful Route 19 from the Canso Causeway). If I had three nights to do the trail, I would spend the first night in Cheticamp having explored the west side of the island, the second night in Ingonish, having travelled the north and north east side of the island, and the third night in Baddeck, having made my way down the east side of the Cabot Trail and experienced the Baddeck area (or, those three spots in reverse, if you choose to travel counter-clockwise).