The three legged dog is a shape made up of 4 notes. The shape looks like a 3 legged dog with 2 front legs, 1 back leg and a tail.There’s a triad on top. You then add another note right next to the bottom note of the triad (this is the dog’s tail on the bottom).
Examples:
DEGB (notice the triad on top)
BCEG (again, notice the triad on top)