3 electron volts
Energy, usually measured in joules, can be calculated as E=QV, where Q is charge, and V is voltage.
Voltage is a measure of how much energy each electron has. It is usually measured in Volts, which are joules per coulomb.
Charge is measured in coulombs, defined as the charge on 6.2415×10^18 electrons.
These are good units to work with for practical electronics. But if we're working with single electrons, those factors of 10^18 make the maths a pain. So we look for some alternative units which might be easier. For charge, it's obvious - the single electron charge. Volts we keep. So that gives us a new definition for energy, from E=QV, the electron volt. 1eV is 1.60218×10^-19J.
So the question boils down to: "what is the energy gain of an electron which has moved to a potential 3V higher". It doesn't really matter where that energy came from, in this case an electric field. E = QV = (1 electron charge) × (3 volts) = 3eV.