Check out the TAG website here! :
If the store isn't working in game and you're signed into PSN, goto the Store from the XMB, then Disc Based Game Add-Ons, and there will be a Rock Band section, RBN section, and RB3 section in the Rs (it separates games alphabetically...I don't remember the exact cut-off but I think it's A-M and N-#s.)
You really just need to go to rbnreleases.com and hit the PS3 link. I'm pretty sure any questions you have and probably will have in the future will be answered there.
Look left, click songs, search for the songs you want, click it, it lists if available on PS3 or not. Example: http://www.rockband.com/songs/UGC_5000492