The three major gases that trap heat in the Earth's atmosphere, contributing to the greenhouse effect, are:

1. Carbon Dioxide (CO2): This is the most significant greenhouse gas produced by human activities, primarily from burning fossil fuels.

2. Methane (CH4): Released from sources like livestock, natural gas leaks, and landfills, methane is a potent greenhouse gas that traps heat much more effectively than CO2 over a shorter period.

3. Nitrous Oxide (N2O): Produced from agricultural activities, industrial processes, and the burning of fossil fuels, nitrous oxide is a long-lasting greenhouse gas that traps heat even more effectively than CO2.

While these three are the most prominent, other gases like water vapor and ozone also play a role in the greenhouse effect.