trouble with root notes

ive got my intro with a chord somewhat arpegiated that goes>
--am--g--f#dim--
does anyone knows(i think everyone knows) what would be the root note or key there,?? i think its the am but i dont know the theorycal way of how it is explained.
and anyway, wat scale fits there??

your song is in G major/eminor normally , starting with am makes it A dorian
correct me if i'm wrong

That sounds about right mightydave, I'd imagine that the A dorian is the more likely scale to be using, seeing as how there are more minor(ish) chords than major in that progression, so using a minor scale is probs. most logical.

Can it also be a key of f#, locrian mode, with F#dim being the i˚-chord, G the II chord, Am the iii-chord,