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