Saliva, tears and nasal mucus have the following in common
1. They are all bodily fluids and keep our bodies healthy and functioning.
2. They all contain antimicrobial substances.
3. They help lubricate and moisten our body parts.

Detailed Explanation

Saliva, tears, and nasal mucus are all types of bodily fluids that help to keep our body healthy and functioning properly.

Antimicrobial Function

One thing they have in common is that they all contain antimicrobial substances that help to fight off harmful bacteria and viruses.

For example saliva, tears and nasal mucus all contain an enzyme called lysozyme which catalyzes the breakdown of the cell walls of certain bacteria. Thus they protect us from these bacteria.

Lubrication

Another thing they have in common is that they all play a role in protecting and lubricating the surfaces of the body where they are found. Like
1. Saliva helps to moisten and lubricate the mouth and digestive tract.
2. Tears help to keep the eyes moist and lubricated.
3. Nasal mucus helps to moisten and protect the nasal passages.

Production

Also, though not an actual similarity, all three fluids are produced by there own different glands in the body.
1. Saliva is produced by the salivary glands.
2. Tears are produced by the lacrimal glands.
3. Nasal mucus is produced by the mucosal glands.