How much NH3 can be absorbed by H2O at 3pa and 0C?

To answer this, you need to know the Henry's law constant for NH3 at this temperature. This constant relates the solubility of a gas to the pressure of the gas above the solution. These constants often have units of M/atm, or some other combination of concentration per pressure units. You might have to do some conversion of the pressure, but multiplying the constant by the pressure will give you the concentration of NH3 in the solution.