[philipkglass]

GPT is not a reliable source. Note that 4 out of 6 of these "molecules with three atoms" it offers do not have three atoms. (Carbon monoxide and nitric oxide have two; iodine trichloride and hydrogen peroxide have 4.)

The GP's somewhat cryptic statement about 3 atoms is explained better here:

https://climatescienceteaching.org/lesson/infrared-spectrosc...

Monatomic gases and diatomic molecules made of identical atoms (e.g. O2, N2) are not "active" with regard to infrared radiation. They are transparent to it. Almost all other molecules are "IR active," meaning that they can absorb IR radiation in ways that are characteristic of their vibrational modes. This includes the well known greenhouse gases carbon dioxide, methane, and nitrous oxide. Greenhouse gases are those IR active molecules that have a significant residence time in the atmosphere and can thus partially trap IR radiation emitted from the Earth's surface.

Some substances are IR active but are not usually classified as greenhouse gases because they quickly break down in the environment. Other substances are IR active but not counted among greenhouse gases because they have a low vapor pressure and do not significantly make it into the atmosphere.

[it]

Fair enough, so 4 of them are not molecules with three atoms.

However, that leaves two that are: ozone (O3) and NO2 Nitrogen Dioxide (NO2). Are you saying these are in fact greenhouse gases?

[philipkglass]

Ozone is infrared active and classified as a greenhouse gas:

https://aura.gsfc.nasa.gov/science/feature-20110403.html

Tropospheric ozone (O3) is the third most important anthropogenic greenhouse gas after carbon dioxide (CO2) and methane (CH4). Ozone absorbs infrared radiation (heat) from the Earth's surface, reducing the amount of radiation that escapes to space.

Nitrogen dioxide is infrared active and absorbs IR re-radiated from the Earth, as seen in the spectral plots available here:

https://vpl.astro.washington.edu/spectra/no2.htm

However, its short atmospheric residence time means that its emissions have little heat-trapping ability, so it is rarely considered as a greenhouse gas:

https://web.viu.ca/krogh/chem302/residence%20time%20of%20atm...