To find the speed of the boats we will divide the distance by time.
From the graph we finde the boats' speeds as ;

And for the speed of the third boat, we will also divide the distance by time.

As you can see the speed of boat 3 (8 km/h) is more than boat 1 (7 km/h) and boat 2 (5 km/h).
Thus the third boat is faster than both of them.
The answer is B.
You can see the speed of boat 1 and 2 in the picture.