The diagram shows the vector e sliding up until its tail touches the head of d.
The sum is the vector from the tail of d to the head of e .
The diagram shows 3D vectors a and b added to form c (the 3D box is there to aid in visualizing the three dimensions). The head-to-tail rule works in 3D as well.
When two vectors are added in 3D, the head-to-tail rule defines a triangle oriented in 3D space.
Is it possible (do you think) to add three vectors together, like a+b+d in the figure?