It would be helpful to draw the system to fully understand the situation. We would see that a right triangle can be made. We can use trigonometric functions so as to relate terms.
velocity, v = 300/5 = 60 m/s
horizontal component of velocity = vcos(theta) = 60cos15 = 57.96 m/s
It will have the same horizontal velocity at the end of 3 seconds which is 57.96 m/s since it is moving at a constant speed. Hope this answers the question. Have a nice day.