Why did the Saudi Fatwa Committee estimate the nisab of grains and fruits to be three kilograms instead of two, and is this not in the best interest of the poor?

The questioner is referring to the amount of the sa' that is obligatory for Zakat al-Fitr, which is estimated at approximately three kilograms. These are religious quantities and are not left to individual or collective discretion. Those who have determined the sa' to be this amount have exercised ijtihad (independent reasoning) in estimating what it was during the Prophetic era. Others may differ in their estimation based on their differing determination of the Prophetic sa'. What appears to be the correct estimation of the Prophetic sa' has been previously clarified in Fatwa No. 94532.