Here's a little theory lesson.
Am is relative to C because they use the same notes. The difference is the tonic (root or I).
As to your ADE, it's a I-IV-V in A so if you play in C you'd have CFG or in G you'd have GCD. All I-IV-Vs, very common progression.
Using the numerals keeps it from being key specific so you want to play I-IV-V in a key then you'd know what chords to play.