The electronic configuration of an element represents the arrangement of electrons in its atomic orbitals. In the case of Cu3, we need to determine the electronic configuration of the copper (Cu) atom.

1. Understanding the electronic configuration of Copper (Cu):
Copper has an atomic number of 29, which means it has 29 electrons. The electronic configuration of Copper (Cu) can be determined by following the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

2. Aufbau Principle:
According to the Aufbau principle, electrons fill the atomic orbitals in order of increasing energy. The orbitals are represented by the principal quantum number (n) and the subshells are represented by the letters s, p, d, and f.

3. Filling the orbitals:
The atomic number of Copper (Cu) is 29, which means the first 29 electrons need to be placed in the orbitals. Let's fill the orbitals one by one:

- The first two electrons will occupy the 1s orbital.
- The next two electrons will occupy the 2s orbital.
- The next six electrons will occupy the 2p orbital (2px, 2py, 2pz).
- The next two electrons will occupy the 3s orbital.
- The next six electrons will occupy the 3p orbital (3px, 3py, 3pz).
- Now, we have filled 18 electrons, and the remaining 11 electrons will be placed in the 3d orbital.

4. Hund's Rule and Pauli Exclusion Principle:
According to Hund's rule, electrons prefer to occupy different orbitals of the same energy level before pairing up. The Pauli exclusion principle states that each orbital can accommodate a maximum of two electrons with opposite spins.

- The 3d orbital has a total of five sub-orbitals (3dxy, 3dxz, 3dyz, 3dx^2-y^2, 3dz^2), and each sub-orbital can accommodate two electrons.
- Since we have 11 electrons remaining to be placed in the 3d orbital, they will occupy the five sub-orbitals as follows:
- The first 10 electrons will occupy the first four sub-orbitals with one electron each, following Hund's rule.
- The last electron will pair up with one of the electrons in the first four sub-orbitals.

5. Writing the electronic configuration:
The electronic configuration of Copper (Cu) can now be written as follows:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸

6. Simplifying the electronic configuration:
To simplify the electronic configuration, we can use the noble gas configuration. The noble gas that precedes Copper (Cu) is Argon (Ar), which has the electronic configuration:
- 1s² 2s² 2p⁶ 3s² 3p⁶

Therefore, the electronic configuration of Cu3 is represented as:
- [Ar] 3d⁸

Hence, the correct answer is option 'A' - [Ar] 3d⁸.