What would be the mass of 3 moles of water molecules?

The molar mass of water is 18.0g / mol (16.0+2x1. 0). Three moles would have a mass which is three times greater or 3 x 18, which you can easily estimate. 3 x 20 = 60, so 3 moles of water will be a bit less than 60.

i think it is true since you can test the answer and can use observations to complete it.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.