Use the least number of times, try to use heavy trucks,

The maximum load of three vehicles is 5T, and at least 8 vehicles are required for 36T (36÷5=7. . 1 7+ 1=8)

Use eight cars. If seven cars are used, the remaining 1T can't be loaded.

If you use six cars plus number three, there are still six tons left. Just use the No.2 car with the No.2 car or the No.8 car.

Therefore, car No.3 was used six times, and car No.2 was used twice, at least (eight times).