The total water received is the area of the valley times the centimeters of rainfall, or (97280 km^2)*(10 cm) = 972800 km^2*cm. To make it easier to compare, convert this to km^3 by multiplying by (1/100000 km/cm). So (972800 km^2*cm)*(1/100000 km/cm) = 9.728 km^3.
Now lets look at the rivers. For their volume we have 1072 km long times 75 m wide times 3 m deep. So that's (1072 km)*(75 m)*(3 m) = 241200 km*m^2. Again, we need to convert to km^3, as we want to compare it with our other figure later. So we multiply by (1/1000 km/m * 1/1000 km/m) = (1/000000 km^2/m^2). This gives us (241200 km*m^2)*(1/000000 km^2/m^2) = 0.2412 km^3. There are three rivers, so the total river volume is 3*0.2412 km^3 = 0.7236 km^3.
So based on what you have written, I'd say they are not approximately the same. However, perhaps there was a typo along the way. If 97280 km^2 is really 7280 km^2, then the rainfall is about 0.728 km^3, which is about the same as the three rivers combined.