Coliform bacteria are a group of bacteria that are commonly used as indicators of water and food quality. They are widely distributed in the environment and can be found in soil, vegetation, and the intestinal tracts of warm-blooded animals. These bacteria are not inherently harmful, but their presence in large numbers indicates the possible presence of fecal contamination and the potential for the presence of pathogenic organisms.

1. Gram-negative:
Coliform bacteria are gram-negative bacteria, meaning that they do not retain the crystal violet stain in the Gram staining procedure. Instead, they take up the counterstain, safranin, which gives them a pinkish-red color when viewed under a microscope. Gram-negative bacteria have a thin peptidoglycan layer sandwiched between two lipid bilayers, making them more resistant to antibiotics and other chemicals.

2. Rod-shaped:
Coliform bacteria are typically rod-shaped, also known as bacilli. They have a cylindrical shape and can vary in length and width. The rod shape allows them to move efficiently through their environment, including water and food sources. Their morphology is an important characteristic for their identification and differentiation from other bacteria.

3. Harmless aerobic microorganisms:
Coliform bacteria are mostly harmless aerobic microorganisms, meaning that they can survive and grow in the presence of oxygen. They obtain energy by using oxygen in their metabolic processes. However, it is important to note that while most coliform bacteria are harmless, some strains can cause diseases in humans, such as Escherichia coli (E. coli) which can cause severe gastrointestinal infections.

In conclusion, the correct statements regarding coliform bacteria are:

1. It is gram-negative.
2. It is rod-shaped bacteria.
3. It is a harmless aerobic microorganism.

Therefore, the correct answer is option A (1, 2, 3).