The three most abundant substances that make up the Earth's atmosphere are:

1. Nitrogen (N2): Makes up about 78% of the atmosphere. It's relatively inert and doesn't play a direct role in most chemical reactions.

2. Oxygen (O2): Makes up about 21% of the atmosphere. Crucial for respiration and combustion.

3. Argon (Ar): Makes up about 0.93% of the atmosphere. An inert gas with little impact on atmospheric processes.