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).

DEGB (notice the triad on top)
BCEG (again, notice the triad on top)