Up to 3 PSPs can connect to the player who is playing MHP3 on the PS3. It doesn't matter whether if you're playing in the same room or via Adhoc Party. Saved files can also be transfered between PSP to PS3 and vice versa.

Also note that to use Adhoc party, your ps3 will have to be connected to your router/modem with wired connection as Adhoc Party will not work with wireless connections. One more important thing is that if your PS3 isn't a japanese PS3 (JP unit not firmware/account) you will not be able to transfer your PSP saved files and configure some PSP Remastered settings, such as using Adhoc Party with MHP3 HD. This is because PS3's firmware 3.70 includes some PSP Remastered functions exclusively for Japanese consoles only at the moment.