How do you download gta mods on a ps3 without a PC?

Answer 1

In order to download you need an internet connection. Without an internet connection even a PC can not download something. If your PS3 is connected to the internet you can download updates to all your Video Games each time you play them and updates are available.

Answer 2

No you cannot. Modding PS and Xbox is impossible. You can't download regular mods into the game. You can mod the game to add simple things such as infinite time on potions or colored writing.