The instantaneous acceleration at t = 3 sec would be zero.
The instantaneous acceleration of a body is given by the slope of the velocity-time graph. At t = 3 sec, the slope of the graph is zero, which means that the acceleration is zero.
Here is an image of the velocity-time graph with the instantaneous acceleration at t = 3 sec highlighted:
The blue line represents the velocity of the body over time. The red line is a tangent to the velocity-time graph at t = 3 sec. The slope of the tangent line is zero, which means that the instantaneous acceleration at t = 3 sec is zero.