Answer:
1/ The change is caused by the phenomenon called “Evaporation”.
2/ No, because evaporation takes place at the surface of water (see Figure below), and since the two glasses have different surface (openings), the rates of water evaporation in the two glasses are different; therefore the remaining volumes of water are different.
3/ Yes, the rate of water evaporation in B is higher than in A, because water surface in B is larger than in A. This means that glass B has lost more water by evaporation than glass A and the water volume remaining in B is smaller than in A.
Water Evaporation Principle
(ResearchGate)