If you have 3 DCAs' on the board counting as one unit and they are each spread out across the board and you had multiple shots and suffered multiple wounds, you would only lose one model if multiple wounds/saves are failed.
All models are called UNITS, even tanks. Sometimes in the literature of the rules throughout the BGB and codex you should keep in mind to pay attention to words like "MODELS", "UNITS" as interpretations can get confusing and combined.