Edit: It looks like songs are not downloadable anymore, which sucks. Thanks, Activision!
This guide should allow you to unlock "Score Biggest," "Score Bigger," and "The Streak" in one play.
I uploaded my own song, called "Score Big w star". The ones I was looking at didn't have Star Power in the songs, so I figured that was just wasting notes. Might as well get the 8x multiplier as well for extra points. I also used two sustained notes (green-red) instead of one.
I did this because I was having issues with Score Biggest, even using some of the songs on GHTunes, especially since a missed note or two meant that I wouldn't hit 3M even after 10 minutes. (Twice I got around 2.7M because I missed a single note.)
Same as other suggestions: one guitar, one drum. Set the drum up on auto kick. Play the guitar, and strum. Use Star Power as you see fit while the auto drum goes crazy.
It's 7:16 in length, so it isn't as long as some others, but you should probably get 3 million at about 5:50. Along the way, the drummer should get "The Streak" if they don't already have it (there are over 6500 drum kicks), so if you're doing this solo, log in on the drums and use the guitar as a 2nd account.
You can also make your own song, as others have suggested. That way you get the "Producer" achievement as well, like I did. :-) You don't have to use it, but I figured since I'd already made it, might as well post it. If it helps you, enjoy.