What does u stand for and could you make your answer a little clearer ?

From the information, it seems the runner is slowing down because the speed at 1st hour is 5km/h but at 3rd hour become 3km/h.

If v= velocity, v0= initial speed, t= time, and the a=acceleration then the function would be:
v= v0 + a * t

To find the acceleration you need to do this equation:
acceleration= velocity1- velocity3 / t3-t1
a = (3km/h-5km/h)/ (3 hour- 1 hour)
a = (-2km/h)/2hour= -1 km/hour^2

After that you need to find the initial speed. Try to put the 1st hour variable into the full equation. It would look like this
v= v0 + a * t
5km/h= v0 + (-1 km/hour^2 * 1 hour)
v0= 5km/h + 1km/h
v0= 6km/h 

Then the full function would be:
v= 6km/h - 1km/h^2 * t

The graph would look like a backslash(\) from 6 gradually go down to 1. 

Gotta be d the music of previous eras