Key of song - 2

Well i never really got what you were saying!!
Do i need to know the key of the song?
Basically ill put down a simple backing track thing, like just G5 A5 Bb5 and loop it, play it on my computer and then jam/solo to it. but i dont know what scale to use! can i use any , Bb, G, A or is there only one? isnt C A minors relative major? so could i use C major too??

Try F major or a G blues scale

G5..................A5 A5
-----------------------------
-13-11----13~15-------10-----------------
------12-------12---------12-10---12~14--
--------------------------------12------12
-----------------------------
---------------------------

=======================================
Bf5
---------8-----------------
-------8---11-8---------------
-10~12---------11 10 8-10-8-------------
------------------------8-8--
--------------------------
--------------------------
that work for ya ?

First light a match, then bend over, Wait , sorry wrong trick, Axl, no matter what chords you are playing, one chord will stick out as an anchor for the progression, if for no other reason than you start and finish with that chord, example, A5-B5-C5, A5 is probaly just going to sound right to end the progression with, it's a natural human response to want to hear that A5 at the end, that's called Resolution, anyhow once you decide what chord is this anchor or Root, you have the key of the progression, now here's the trick, take your chords, write out the notes for every chord in the progression, write out the notes in order starting with the Root note and you will have a scale that sounds fine over those chords, ie.. chords D-G-A, notes of the chords, DF#A, GBD, AC#E, Scale notes D-E-F#-G-A-B-C#-D, this is simple I admit, but it's a good place to start

G5 = G, D
A5 = A, E
Bb5 = Bb, F

G, A, Bb, D, E, F

Now, you have the basis of the scale from the chords.
It's going to be some kind of G minor scale/mode from the Bb (flat 3). You can throw any kind of C you want in there and make up your own scale, since it's not implied by the chords, or you can pick a mode.

You have the 1, 2, b3, --, 5, 6, b7. The obvious mode is Dorian, since you have a b3, b7, and 6, so that makes the missing note a 4, which is C.

Depending on how you play the chords, however, you may want to choose another mode. If you're going the heavy metal route you may want to play G Phyrgian, Aeolian, or Minor Pentatonic. Aeolian or Minor Pent. for the rockers, Dorian for the jazzers, or Phyrgian for the Spanish sound. It's all in what sound or mood you're trying to capture.