Hey there, first time poster here...

I've heard through the grapevine that there was a way to upload or import MP3s into RPGM3... any truth to this rumor? If so, how would that be done? It would be great to add third party BGMs!!

No

On Page 3 of List everything you want to know about RPG Maker 3! I asked if I could import in sounds, textures, or images and was told no.

Maybe next version on the PS3.

It's not going to happen precisely because of the copyright mess Agetec could face if they allowed MP3 support in the RPG Maker games.

In RPG Maker 2, a Midi-to-RM2 convertor was created by ValhallaRPG. But that was a fan-created program. Besides, there's nothing much to modify in RM3.

Maybe the ability to make custom music will be in RM4.

Yes, I'm sure Sony still owns the rights to many samples from their synthesizer and keyboard line. I'm sure Agetec could ask for those to be available in the game since it would be making Sony Music on a Sony Playstation.

Or they could go with more themes.
Celtic: Dulcimer Hammer
Celtic: Drums
Japanese: Taiko Drums
Japanese: Harpiscord (forgot the name, sashima,... no forget it)
India: Sitar
India: cymbals

Just an example. They could load up a beat and tempo and you could mix them. That would rock. I haven't found the music yet in my game.