answering your second question. Not easy...Keeping it simple - for most basic rock/blues, you'll have the 3 chord progression you describe. Such as A-D-E or E-A-D etc. Normally, if you're playing blues for instance, you have the 12 bar blues in "A", which would be your A-D-E chord progression, Blues in "E", E-A-D, so on. Of course, there's a hundred variations, but the most common are A and E, for blues, (and AC/DC LOL). In these cases they're calling the first chord the "key". If you're listening to a Dream Theater song...the key and time signatures will change with each breath....I need to go back to music school just to listen to them.