if you're playin 5-1-3, power chords, thats an A5-F5-G5 (the 5 just tells you that its a power chord) The key this would be in, since you're starting on an A would most likely be A-minor. So if you're soloing, you can either use the a-minor scale, or other offshoots of that, the modes, with the ionian mode starting on A, or the pentatonic scales, with the first pattern starting on A. Most of the time, the first chord in a song tells you what key its in, and most of the time, it'll be so the pentatonic scales start off that note (the root of the first chord). Hope ive been of some help, and if i can clarify anything, let me know.