[AL phy]electric potential

Please remind that electric potential needed to be defined with a reference point, in the case of kQ/r, we set the potential is zero at r=infinity. In your question, a conducting sphere is charged to a potential of 3V, mean that with a reference potential that waas no noticed, the potentail diffrent between the conducting sphere with that reference is 3V. The omitted reference point here should be set at r=infininty, potential there is zero.
Potential is defined by the energy gain/loss of a particle moving in a field, that could be an EM (electromagnetic) field, or a gravitation field. P=integrate F ds, where F is the force acted on the particle by the field, s is the path the particle move in the field. The equation kQ/r is the potential different between infinity and r, with an EM force by a point charge/ sphere, where r is larger the the radius of the sphere. Force by this field is kQ/r^2
Taking an example, if I charge up a very long straight wire, the EM force with be kQ/r, and the potential with be kQ ln(r/ro), with reference point at ro, potential there is set to be 0 (but not zero at infinity). If you know calculus, you will have a clearer picture.

Any charge could raise the potential in the space surrounding it. If charges are put onto an obejct (say, a metallic sphere), the electric potential of the sphere will be raised from the original zero volt to a certain voltage (like 3V as in your question).

At the same time, elctric fields are produced at the vicintiy of the sphere. As field lines run from places of higher potential to places of lower potential, the existence of electric field lines indicates a distribution of potentials surrounding the charged sphere.

The equation 1/4兀e x Q/r quoted by you is one used to calculate the potential at a distance r from the centre of a charged sphere.