Introduction:
Optical isomers are stereoisomers that have the same molecular formula and connectivity but differ in their spatial arrangement. [Co(en)2Cl2] is a coordination compound in which the central cobalt atom is coordinated with two ethylenediamine ligands and two chloride ions.

Determination of optical isomers:
To determine the number of optical isomers, we need to consider the presence of chiral centers in the molecule. A chiral center is an atom in a molecule that is bonded to four different groups. In [Co(en)2Cl2], the two nitrogen atoms in the ethylenediamine ligands are chiral centers.

Calculation of possible isomers:
The possible number of optical isomers can be calculated using the formula 2^n, where n is the number of chiral centers. In this case, there are two chiral centers, so the total number of possible isomers is 2^2 = 4.

However, not all of these isomers are actually possible due to the presence of a plane of symmetry in the molecule. A plane of symmetry is an imaginary plane that can be drawn through a molecule so that one half of the molecule is the mirror image of the other half.

Elimination of impossible isomers:
In [Co(en)2Cl2], there is a plane of symmetry that passes through the central cobalt atom and bisects the two ethylenediamine ligands. This means that the two possible mirror images of the molecule are superimposable, and therefore not true optical isomers. The remaining two isomers are:

- Enantiomer 1: The two ethylenediamine ligands are arranged in a clockwise manner around the cobalt atom.
- Enantiomer 2: The two ethylenediamine ligands are arranged in a counterclockwise manner around the cobalt atom.

These two isomers are true optical isomers, as they are non-superimposable mirror images of each other. Therefore, the possible number of optical isomers in [Co(en)2Cl2] is 2. However, we must also consider the fact that the chloride ions can occupy two different positions in the molecule, resulting in a total of 2 x 2 = 4 possible isomers. Out of these four, only three are actually possible due to the presence of the plane of symmetry. Thus, the correct answer is 3.