What is the unit for resistance, voltage and current (all three separately)

The unit of resistance is the ohm.  Resistance is a measure of the energy
that a component robs from current passing through it, and dissipates.
A resistance of 1 ohm means that in order to force 1 Coulomb of charge
(6.25 x 10¹⁸  electrons) through that obstacle, you need to give it 1 joule
of energy.  The electrons will lose that much energy on their way through
the 1-ohm "resistor", and the joule will make the resistor warmer. 

The unit of electrical potential difference between two points is the volt.
1 volt of potential difference means that in order to lift  1 Coulomb of
charge (6.25 x 10¹⁸ electrons) to a place where the potential is 1 volt
higher, you need to give it 1 joule of energy.  And when the Coulomb
falls down through a circuit to a place where the potential is lower, it
gives up 1 joule of energy for every volt it falls through.

The unit of electric current is the Ampere.  A current of 1 Ampere means
that  6.25 x10¹⁸ electrons (1 Coulomb of charge) are flowing past a point
in the wire every second.