With the information provided, average speed is really the only thing we can calculate; we have no idea if the car maintained the same speed during the entire 3 minutes, or if its velocity changed significantly at any time therein.

The average, in this case, would refer to equal partitions of distance and time, such that the total of 5km equals 3 minutes, and vice versa. We could represent this as;

5/3 = 1 2/3 km per minute, or

3/5 = 0.6 minutes per km

In physics we typically use seconds as the default measurement of time, and meters as the default measurement of distance; so we need to do some conversions.

5km x 1000m/km = 5000m

3 minutes x 60seconds/minute = 180 seconds

5000m/180s = 27.78 m/s

180s/5000m = 0.036 s/m

We could also convert to show the average speed of the car in the more familiar form of km/hr

3 minutes x 1 hour/60minutes = 0.05hr

5km/0.05hr x 20 (to set 0.05hr equal to 1)

100km/hr.