when you are finding displacement from a velocity graph
and it says find the displacement at t=3
do you find the area from the points 0 --->3 to the x axis from 0--->3 or no?
The area below a velocity-time graph between two instants of time t is the distance travelled. The signed area below the same graph is the displacement.
Therefore the period 0 →3 sec. is correct if this is what your question or teacher wants. If the question asks for the distance travelled between time t=2 and t=3, then it would be the area between those two times.