Unfortunately, doing this kind of problem by hand would take a very long time. So I recommend using a graphing calculator. Specifically, one that can handle numeric integration. You'll need to find the area under the curve from t = 0 to t = 3. This will tell you the change in velocity during the time period from t = 0 to t = 3 seconds. You should find that
If you want to do this by hand, you can use Riemann rectangles to break up the area and find the area of each rectangle. Though use of a calculator is recommended here as well.
This result will add onto the initial velocity of 5 to get a final approximate result of 5+6.71005 = 11.71005 which rounds to 11.710