system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When is 1, 2 ,3 gears in an AUTOMATIC car used?

I always use D when I use an automatic. 

Every time I ask someone about the 1,2,3 gear I get a different answer.

Anyone can explain with justifications and sources pleasE?
gpt
Transmission mechanic here. When you shift your vehicle into 1, 2, or 3 you are manually shifting the transmission into said gear. 1 will put it in first gear and 2 or 3 will put it in the optimal gear based off vehicle speed, engine load, etc up to that gear you selected. This will allow you to have more torque to your wheels as you are in a lower gear. This helps you if you are trying to move a heavy load or if you are going down a long downgrade. Selecting one of these gears will help you slow down as you are causing engine braking. Do not use these selections if you are going above 30 as it will burn out your transmissions Hope this helps.