物理(熱和氣體)

Statement (1) is right.
If the gas is not completely dry, water vapour in the flask would also exert pressure. Thus, the pressure measured by the pressure-gauge is higher than the pressure of the gas. This gives error to the result.

Statement (2) is right.
Gas is a bad conductor of heat. The thermometer only measures the temperature of water in the water-bath. Hence, there might be a difference between the actual temperature of the gas and the temperature recorded by the thermometer.

Statement (3) is wrong.
Thre is no temperature change when the water vapourizes. This doesn't affect the reading of the termometer.