To do this, you need to make sure that all speeds are the same unit. Since there are three 2 speeds already in km/hr, it would be best that you match the other 2 speeds into that unit as well.
Conversion of 15,000m/hr:
time is already in hours so you just need to make convert the meters to kilometers.

Conversion of 4.3m/s

Now that they are all the same value, you can see that Fred at 4.3m/s was the fastest.