Yes, that's exactly what I meant. So in the key of C you would have your three chord trick of C, F, G and the relative minor is Am. If you consider the notes in the C major scale, you have:
1 2 3 4 5 6 7 8
C D E F G A B C
so the relative minor (Am) corresponds to note 6.
I should state again that the theory is extremely useful, but in my personal experience it was only useful once I could play a few songs confidently. I'm currently attempting to learn pedal steel and no amount of theory knowledge will compensate for a lack of physiological skill in playing the thing! The two go hand in hand, but I'd still advocate learning the 'how' before you need to know the 'why'

Do you play any other instruments, just out of interest?