It is actually possible to install mods for Fallout 3 on your PS3, but you'll have to run your own search and figure it out. I don't want to take responsibility for posting any how-to links as I'm not clear on the forum rules in regards to console-hacking.
Honestly, I picked up a used computer that was surplus from my city's municipal government offices for about $100 Canadian, and it was good enough to play Fallout 3 on. If you can do the research on what you need and where you can get it cheap, it is possible to build a very thrifty gaming PC that'll handle games as old as Fallout 3 is now.