The object starts from point A, travels to B which is 150 m away and returns to A in 3 sec. Now velocity is the total displacement divided by the time taken. As the object travels from A and returns back to A, there is no displacement. Therefore the velocity of the object is 0. Assuming that the object started at rest from A, the final and initial velocity of the object are the same, i.e. 0 m/s. Therefore there is no acceleration.
From the information provided we can calculate the speed of the object which is distance travelled by time taken or 300/ 3 = 100m/s.