Step 1: Use the ratio of speeds.
Let the distance be \(d\). Time upstream is 3 times time downstream, so speed downstream : speed upstream = 3 : 1.
Let the upstream speed be $u$ and the downstream speed be $3u$.
Step 2: Use the still water speed.
Speed in still water is the average of the two speeds:
\[ \frac{3u + u}{2} = 17 \]
\[ 2u = 17 \]
\[ u = 8.5 \]
Step 3: Find the stream speed.
Stream speed is half the difference of the two speeds:
\[ \frac{3u - u}{2} = u = 8.5 \text{ km/hr} \]
So the stream speed equals the upstream speed here.
Step 4: Match with the options.
The value 8.5 km/hr is option 3. The other values 15, 11.5 and 6 do not give a 3 : 1 speed ratio, so they are not correct.
Final Answer:
The speed of the stream is 8.5 km/hr.
\[ \boxed{8.5 \text{ km/hr}} \]