To determine which of the given compounds have three stereoisomeric forms, we need to analyze the coordination complexes and identify the factors that contribute to stereoisomerism.

Let's examine each compound individually:

i. [Cr(NO3)3(NH3)3]
- This compound contains a chromium ion coordinated to three nitrate ions and three ammonia molecules.
- The chromium ion has a coordination number of 6, and its geometry is octahedral.
- In an octahedral complex, there are two possible types of stereoisomerism: geometrical isomerism and optical isomerism.
- Geometrical isomerism occurs when there are different spatial arrangements of ligands around the central metal ion. However, in this compound, all the ligands are the same (either nitrate or ammonia), so there is no geometrical isomerism.
- Optical isomerism occurs when the complex has a chiral center, which requires the presence of different ligands. In this case, all the ligands are the same, so there is no optical isomerism.
- Therefore, this compound does not have any stereoisomers.

ii. K3[Co(C2O4)3]
- This compound contains a cobalt ion coordinated to three oxalate ions.
- The cobalt ion has a coordination number of 6, and its geometry is octahedral.
- In an octahedral complex, there are two possible types of stereoisomerism: geometrical isomerism and optical isomerism.
- Geometrical isomerism occurs when there are different spatial arrangements of ligands around the central metal ion. However, in this compound, all the ligands are the same (oxalate ions), so there is no geometrical isomerism.
- Optical isomerism occurs when the complex has a chiral center, which requires the presence of different ligands. In this case, all the ligands are the same, so there is no optical isomerism.
- Therefore, this compound does not have any stereoisomers.

iii. K3[Co(C2O4)2Cl2]
- This compound contains a cobalt ion coordinated to two oxalate ions and two chloride ions.
- The cobalt ion has a coordination number of 6, and its geometry is octahedral.
- In an octahedral complex, there are two possible types of stereoisomerism: geometrical isomerism and optical isomerism.
- Geometrical isomerism occurs when there are different spatial arrangements of ligands around the central metal ion. In this compound, the two oxalate ions can be arranged in cis (same side) or trans (opposite side) configurations, resulting in two different geometrical isomers.
- Optical isomerism occurs when the complex has a chiral center, which requires the presence of different ligands. In this case, all the ligands are the same (either oxalate or chloride), so there is no optical isomerism.
- Therefore, this compound has two stereoisomers.

iv. [Co(en)2ClBr]
- This compound contains a cobalt ion coordinated to two ethylenediamine (en) ligands, one chloride ion, and one bromide ion.
- The cobalt ion has a