The correct answer is option 3: 1, 2, and 3.

Option 1 states that the size of farms in commercial grain farming of the middle latitude grasslands is generally large. This is a characteristic of commercial grain farming because large farms are needed to accommodate the extensive cultivation required for growing crops on a commercial scale.

Option 2 states that cultivation is highly mechanized in commercial grain farming of the middle latitude grasslands. This is also a characteristic of commercial grain farming because mechanization helps to increase efficiency and productivity in the cultivation process.

Option 3 states that commercial grain farming of the middle latitude grasslands is a type of extensive farming. This is true because extensive farming involves the use of large amounts of land with minimal inputs of labor and capital. Commercial grain farming fits this description as it requires large areas of land for cultivation, but typically involves limited inputs of labor and capital per unit of land.

Therefore, the correct answer is option 3: 1, 2, and 3.