Answer:
ISO 1600
Step-by-step explanation:
"Three stops" means the picture is overexposed by a factor of 2^3 = 8. Each f-stop increase halves the exposure, so increasing from 5.6 through 8 to 11 means you have cut the exposure by a factor of 4 with the f-stop change.
Each halving of the shutter-open time also cuts the exposure in half, so reducing the shutter speed from 1/30 to 1/250 cuts the exposure by a factor of about 8.
The adjustments of shutter speed and f-stop have decreased the exposure by a factor of 4·8 = 32, which is a factor of 4 more than the factor of 8 you needed. (The shutter speed change by itself would have been sufficient.)
So, to compensate for the much smaller exposure, you need faster film, by a factor of 4. The film needs to go from ISO 400 to ISO 1600.
