It's tricky to define "most commonly used" as it depends on the context: industrial use, household use, or overall global production. However, considering a broad perspective, here are three gases commonly used worldwide:

1. Nitrogen (N2): This is the most abundant gas in the atmosphere, making it readily available. It has numerous applications:

* Industrial: Inert gas for manufacturing, food packaging, welding, and chemical production.

* Agriculture: Used in fertilizers.

* Medical: Used in cryosurgery.

2. Oxygen (O2): Essential for life, oxygen is used extensively:

* Medical: Respiration, treating respiratory problems.

* Industrial: Steel production, welding, and chemical processes.

* Environmental: In wastewater treatment.

3. Carbon Dioxide (CO2): Although often viewed as a greenhouse gas, it also has beneficial uses:

* Food and Beverage: Carbonation in drinks, food preservation.

* Industrial: Used in manufacturing, fire extinguishers, and lasers.

* Agriculture: Used in greenhouses to promote plant growth.

Important Note: Other gases like hydrogen (H2), helium (He), and natural gas (primarily methane) are also widely used, depending on the specific application.