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)