What are three ways to change the density of a gas?

Density of a substance is the ratio of its mass to its volume. That is,

density = mass/volume.

Thus, one way to change the density of a substance is by changing its volume. If the volume is decreased, say by compression, the density will increase. And an expansion will, similarly, cause the density to decrease.

Changes in temperature and pressure will cause changes in the density of a gas. When a gas is heated, its molecules gain more kinetic energy and move farther away from each other. This results in a less dense packing of molecules and hence lower density. Materials contract when frozen (water is an exception) and hence will have more density. Similarly, we can pressurize a gas and confine it to a smaller volume. This will cause an increase in density, as more molecules are present in the same volume.

Thus, changes in volume, pressure and temperature can cause density changes. Note that change in one of these factors may cause another factor to change as well.

Hope this helps.