To run a downloaded .iso on a real PSP, you need Custom Firmware (CFW) – e.g., PRO-C or LME Infinity.
Quick tutorial:
Troubleshooting: If the screen goes black, disable all plugins in the Recovery Menu (press Select on the XMB → Recovery → Plugins → Disable).
Unlike the PC version which relied heavily on "Opportunities," the PSP version of The Sims 3 introduces a Karma System. This mechanic is the core of the handheld experience.
If you own the UMD, you can legally play it on the PPSSPP emulator by creating your own .iso. PPSSPP runs The Sims 3 beautifully on Android, iOS, Windows, and Linux.